I have beaten the game on Normal, Nightmare and Hell and have started doing some stuff in Inferno (I've hit act II and got trainwrecked by the first white mobs). The guys who found the exploits and have no lives have ruined the high-end economy. Not really blizzards fault, other than letting the exploits exist in the first place. They have basically stated that they will nerf the best classes and exploits until they are all classes are about equal. That doesn?t prevent the people who have already exploited those things to stay ahead of the game.
I did buy and enjoy the game. I also think it needs a lot of work.
The game is highly flawed at the higher difficulties. Melee classes can't progress (I started as a barb and had to switch to a wizard) because they require gear that is being farmed by the ranged classes that can progress and get sold at unbelievable prices. Upgrades for my barb cost 1M+ and it takes about an hour to farm 100k. Good weapons are 2M+ if you're lucky to snipe an item off the AH at the right time.
You can't farm loot because the loot is soooo bad. Lucky to get 1 piece of AH-able loot completing an entire act with the Valor buff. This is because items NEED a certain combination of stats to be useful and it's nearly impossible for those stats to randomize on a rare. If they do randomize, the numbers are low making it useless. Legendaries are mostly just bad.
Blizzard and others claim that there is no "cookie cutter" spec. But, when you reach the later acts in hell you end up with a "cookie cutter" spec because there's no other way to survive. It's pretty fun the first two playthroughs on a toon - when you can actually choose the abilities you like to use. But to get the best loot and play Inferno your build will revolve around a couple of key abilities that won't change. Just hit the D3 forums to see for yourself.
Laggy servers - mobs can often one shot you at higher difficulties. Nothing is as frustrating as getting melee'd by a mob that swung AFTER you teleported all the way across the screen. It happens. A LOT.
Multiplayer is BASICALLY REQUIRED. Yeah, it can be done solo (this is why the first two playthroughs are so fun). But it stinks because elite/champion packs will regen to full hp if they aren't in combat. They also do a ton of damage and can have some horrible affixes that can require you to either skip them or have to restart the level. Restarting a level or skipping means potentially losing your Valor stacks.
Public games are the best source of the worst players you could possibly imagine. Have friends that play or go solo.
The AH is your main source of loot. No more finding that epic item and being like "OMG guys, look at this leet item that dropped!!!" Just take your mediocre loot to the AH, hope some scrub buys it and that it will pay for a real upgrade.
Some Champion/Elite packs are nigh unbeatable in Inferno.
Gems aren't very interesting (compared to D2).
RMAH is going to take achievement out of the game. Just RMAH that epic loot because the game is brutal in Inferno.
Looks freaking sweet. Abilities are stylish and look like you're actually doing something. Artistically it's very impressive.
Story is pretty good, if you don't pay attention to the details. Tyrael is bampf.
Really fun to play when it's not lagging and before you get to late Hell.
Solid hack'n'slash gameplay.
Rune system is interesting.
Classes are varied and make rerolling fun.
Champion/Elite packs that are beatable are nice an challenging.
TL;DR version: Worth it if you have 60 bucks and time to burn. But, don't expect to get to the top anytime soon unless you find an expliot. If you can wait, I think Torchlight 2 will be more entertaining.