This is my first review and it is quite long, so please bear with me.
To make it easier to shift through the wall of text, I have broken the review down to several sections.
So, here it goes:
Graphics and Sound: They are both quite great, especially the music deserves special mention.
The environments are beautifully designed, the spell effects are flashy and the environmental effects are also great. The colors are maybe too vibrant but that is a design decision and subject to individual taste.
Story and atmosphere: Perhaps my greatest gripe with the game. Anyone who has played other Larian games probably knows that their stories range from "meh" to somewhat decent. I've never really been bothered by this. While I do like it when a RPG has strong story and character development, as long as the game mechanics are good, I can live with a generic plot.
Unfortunately, the story in D:OS is so bad that it actually diminishes my experience. The plot itself is so unremarkable that it gives you no motivation to move forward. You can probably stop half-way through the game, come back to it in a few months, remember nothing and, still, get the same feeling (the feeling is "meh").
The characterization of most NPCs is just laughable and the companions, well, I will only say that I should have probably gone with the generic henchmen. At least I would have been spared the pain of listening to their ridiculous remarks.
As far as the atmosphere goes, I should mention that Larian games never take themselves particularly seriously and you can usually find several pop-culture references and plenty of humor. However, this time they seem to have gone out of their way to destroy any sense of immersion. Most of the NPC dialogs are filled with completely useless information that do not add anything to the world lore or main plot. They are just lame joke after lame joke and if you manage to filter through all that you will finally get something useful (Don't get me wrong, some of the jokes really had me laughing out loud, but in most cases they are just trying way too hard).
As a final note on the story, I would like to mention that the whole premise is taken from the Wheel of Time series (Source, taint, source-hunters=Aes Sedai). Now, if the actual story of the game was any good, I would probably regard this as an amusing reference to the books, but as it is, it just looks to me like lazy writing.
Combat: By far, the greatest strength of D:OS. The use of environmental hazards makes even the trivial encounters quite fun. My main issue with the combat system is that your opponents have fairly lively animations which has caused my, on multiple times, to click on the ground and waste my turn. This could have easily been avoided if the bounding box was larger or if there was a confirm stage after each move.
Character creation: Another strength of D:OS. There are plenty of skills to choose and the classless system makes any combination possible. Unfortunately, as the game progresses, you will notice that most of the skills are repeated among skill groups (i.e.There at least three different skill groups with a talent that sets "bleeding" status on an enemy. There are also skills that are simply useless). Still the number of unique skills remains quite large and can suit different play-styles.
Interface: My other great issue with D:OS. Handling items is extremely tedious. The inventory is almost as bad as that of Neverwinter Nights. There are tabs to organize items by type, but if you have a mule character then prepare for endless frustration. Crafting, repairing and identifying items could and should have been much easier to use too.
Also, there are no quest markers (some indicators have been added with the latest patch, I think, but that's it). In my opinion, this is a good thing. It encourages exploration, keeps you involved with the game world and even if you skip all the dialogs, your journal will pretty much point you to your next destination.
A couple of quests are a bit vague, but all in all it doesn't really make the game any more difficult. There are people who seem to find the lack of markers off-putting but this isn't skyrim (as in huge world map). The size of the maps is easily manageable.
Gameplay: I have read several reviews and an overwhelming amount of comments by people who say that D:OS is huge, open and with endless sub-quests. This is simply inaccurate. As I am writing this review I am nearing the end of the second map (3 in total). The first map is rather large (roughly the size of two quest areas in BG2) and in theory, you can go anywhere you like.
However, if you don't do all of the quests in an area and not follow the path picked by the designers you will end up severely out-leveled (not to mention that most of the sub-quests are tied to the main plot and any related dialogs will make no sense if completed out of their intended order). This is a glaring design flaw, in my opinion, since sub-quests are not really optional, that way. Also, the concept of exploration is somewhat diminished, since, in the end, you will actually have to go through every area in order to progress the main plot.
Unfortunately, things seem to fall apart in the second map. Any sense of freedom vanishes as soon as you set foot in the forest and the game railroads you from encounter to encounter.
The difficulty, also scales quite badly. While the first map can present some challenging battles, once you hit level 9-10, most of the battles become trivial (I should mention I am playing on Hard). Even encounters with multiple enemies 2 levels above your own can be easily dealt with, without using any overpowered skills or even any particular planning.
The loot system is quite bad as well. Not sure how it works (I think it is mostly random, apart from some unique items), but most of my characters were stocked with legendary items by the end of the first map.
I have reserved the end of the gameplay section for one of the most ridiculous game mechanics I have ever seen in a game: The dialog conflict resolution system.
All conflicts among characters have to be resolved through a mini-game of rock-paper-scissors. Your character's social skills determine how many times you have to win the game in order to win the argument, so at least there's that. But as a mechanic that is being used over and over, it is just STUPID. If it was used once or twice I would probably have perceived it as a smart joke on the dice system (roll a number, add your skill) that is used in some RPGs (mostly D&D based). But it's not a joke. It's just BAD.
I should also mention that the fact that you have to resolve conflicts among your 2 main characters (the ones that you created) is a ridiculous concept in single player.
Replayability: Not much, I'd say. The only reason to replay the game would be to try different combat skills. Story-wise, there are no meaningful choices to be made. You will also most likely have seen all of the areas in your first playthrough.
I should also mention that the game is not as long as people make it out to be. While it might take up to 50 hours to finish, this time is mainly occupied by the turn-based combat (even a trivial encounter could take a while to resolve)
Conclusion: The review might seem to focus on the negative aspects of the game, but this is only because I wanted to address the unrealistic expectations that have been created by, what I consider, undeserving overwhelmingly positive reviews (reviews taht made me decide to get the game sooner than I had actually planned).
I would like to clarify that I consider the game quite enjoyable, as a whole, and every cRPG fan should try, if only to support the devs and show that there is an audience for this type of games. However, while D:OS is a step in the right direction, it would be wrong to give it a free pass simply because pretty much all RPGs that have come out since DA:O are barely worthy of the title.
Tl;DR: If you like cRPGs, it is definitely worth your time (if not the full price. Maybe wait for a 20% discount if you still have other games to play). Younger gamers that want to have a taste of how RPGs were 10 years ago (better than they are now, in case you are wondering) might also want to give it a try, since I think it is a good entry point.
Do not expect, however, to find the new BG or Ultima or even Temple of Elemental Evil (post community patch of course).
EDIT: After finishing the game last night, I felt that I had to lower the score to 6/10. Phantom forest is even worse than the second map (though, still, beautifully designed). It mainly consists of wandering in the forest,
killing stuff that by that point present absolutely no challenge. There are a couple of nice touches, like the entrance to the temple of the dead, but by that time, the broken journal, the horrible UI that kept annoying me all through the game and the ridiculous enemy AI had become too much.
Unfortunately, after the first map, the game becomes too tedious. I only managed to slog through it because I generally enjoyed the combat mechanics and hoped that things would actually get better.