Excessively fun but with some annoying issues
First things first, Titan quest is well done both graphically and aurally, with decent texturing, nice SFX and even grass swaying when you move through it. The music is suitable to the game and the ambient sounds are enjoyable without being intrusive. Some people complain about the voice acting but believe me, there is much worse out there (the alien voices in Nexus: The Jupiter Incident spring to mind)
Another nice touch is the physics - when a creature dies or an object falls, it bounces and flops around in a relatively realistic manner (although you do get quirks such as bodies getting stuck on the ends of trees), and it can be extremely fun to use an explosive spell on a group of enemies and watch them fly.
The gameplay is pretty much what you'd expect and excellent in it's execution, with the monsters, items and stats balancing out well for each section of the game. Also, like in Diablo 2, you get"boss" and "hero" monsters which are extra tough or difficult but provide added experience points or loot when slain.
Now for the bad.
Although my system admittedly isn't the most powerful, it should be able to run the game with little difficulty on medium settings. Unfortunately, the frame-rate is choppy most of the time and although it can be ignored, it does detract from the enjoyability of the game. I have talked about quirks in the physics model, but I actually consider those to be amusing and believe they add rather than detract from the game. The game is rather buggy - or was, should I say - since when I played the (un-updated) demo there were texturing and gameplay bugs horrible enough to make you smash your keyboard. Fortunately, once you have the full version and update it the problems seem to disappear.
Overall I'd suggest this game for anyone who is a fan of the Hack 'n Slash genre, it may not have the role-playing of games like Baldur's Gate, but it makes up for that lack in sheer mindless entertainment.