An excellent game for the hard-core role player but a bit of a rough ride for the casual gamer.
The closest comparable peer to G2 is Morrowind with which G2 shares some of the broader characteristics. While not quite as large as Morrowind, G2 takes place in a very large open landscape across which you are free to roam and explore. The graphics are a bit behind those of Morrowind but the artistry put into the game makes G2 far more visually satisfying that Morrowind and even Oblivion (whose environment has too much of a clean and constructed feeling to it).
A particular note is in order for the Night of the Raven expansion pack which adds a new region to the game and some of the parts of this new region are simply outstanding. The swamp area is thick with tall plants and trees and a huge desert canyon provides a breathtaking experience.
The main area of G2 that you play in is heavily forested and is just covered in plants and trees. The level of detail here is just amazing and beats out Morrowind which didn't have a lush feeling to it. You can follow paths through the forest where occasionally you can see that part of it has been dug through a small hill or deep wheel ruts carve away at the ground.
There are three large areas you get to play in G2 (if you include the Night of the Raven expansion pack). The most striking thing about each of the areas is the complete lack of loading zones (except between the three areas) which is something not even Morrowind or Oblivion achieves. Each cave, each house, each town and each ruins you come across are all part of the same map that you can simply walk through without missing a beat. This continuity adds to the immersiveness of the game and really makes you feel like you're really poking around in a real cave or walking into a real town. That said, one thing somewhat missing are "dungeons" in the traditional sense of the word. The caves and ruins you explore are almost all extremely short and generally consist of one or two rooms and never more than ten or so. Almost all of your experience takes place on the outside.
Perhaps the most important aspect of the game to decide if you'll enjoy it or not is the difficulty of the combat. When you first set out in the world, nearly every creature you come across is more than likely to kill you numerous times. You will find yourself saving and reloading very often. G2 is very much a game of killing one monster at a time and since there are a lot of monsters in the game, you will probably find G2 to be one of the longest games you've ever played.
As many other reviewers have noted, the combat system is a bit clumsy. You have to hold either the CTRL or mouse button and press one of the movement keys to attack, block or cast a spell. Some spells don't seem to work at close range and as you attack forward, you also move forward which sometimes puts you on top of or beyond the monster you're fighting. There is also some player skill needed in melee combat as the timing of your attacks can counter the attacks of your enemy. If you can get into the right rhythm of attack, you can slowly take down a difficult enemy. Of course, if you find yourself up against two enemies, then you're in big trouble because you can only counter the attacks of one of them while the other one rips you to pieces.
The leveling and skill system is fairly straightforward. You have a moderate number of skills that you can improve over time. Each time you level up, you're granted a number of learning points which you can then use to improve your skills or learn new tricks such as creating potions, learning spells, learning to lock pick or learning how to collect various parts from a dead animal. One of the weaknesses of the system however is that the number of learning points required to increase a skill increase as the skill does. Since the number of learning points you gain is fixed per level, it becomes more and more difficult to improve your character later in the game which tends to add a slowing down feeling to it.
And of course, this review cannot be complete without giving credit to the quests provided in the game. As in games like Morrowind, this game throws at you quite a handful of various side quests that provides for a very open-ended and full playing experience. In addition, you'll find that many of the quests are quite a challenge to complete. You are often only given incomplete information about where to go and quite a number of quests give you no information at all and you have to simply explore the entire island to find what you're looking for. There is no auto-map that fills in locations of places as you visit them so you'll be left on your own there.
Finally, the interfaces used in the game are simply terrible. Fortunately, it is something you can live with but I can't think of any game in the last ten years with an interface as boring and uninspired as the one in G2. Every dialog and menu in the game is entirely text based with a plain, single line border around it. There is no support for a mouse cursor although you can use a mouse wheel to move up and down the menus. The inventory page is just a grid of small images with equipped items highlighted and there is no rag doll used for equipping items.