The best Diablo clone on the market
Titan quest is great fun to play, although it is very long and does get very repetative, but thats why its called Titan quest and not Mediocre quest. The interface is very user friendly, all spell casting is a breese so your not stuffing around with every button on the keyboard. Once your spells are active just keep slash 'n' hackin' away. Graphics are top notch. Sound is pretty good and the story is also pretty good.
But where the game excels is in the amount of treasure there is to pick up. 1000's of items scatter the countryside, with so many cool weapons, armour etc to find and customise. This is what compells you to go on, not the story. Because you always want that better sword or shield that you might just get from your next kill or chest! (Although warping back to shops to sell goods becomes a bit too regular, you'll be thankfull later on for all the gold you have)
And Because the game is so easy to play, you could stop playing for a few months, then start right up again where you left off with no real thought of "crap what do I do again"
If your a Diablo nut and just can't find anything to fill the void waiting for Diablo 3 then this is the best clone by far i've found. Worth playing just for the cool gear you find!