Titan Quest, and its worthy expansion Immortal Throne is, imo, part of the holy trinity in ARPG's along side Diablo and Dungeon Siege. Titan Quests theme is unique, and the gameplay is so faithful to the principles that made diablo so great.
There is a massive, diverse world covered in several acts. In combination with tons of loot in the suspected qualities to keep the flow of the game intact. On top of that, the game also follows the model of increased difficulty (normal, epic, legendary), remeniscent of Diablo's normal, nightmare and hell. Each difficulty features harder monsters and better loot.
If at one point, you would get tired of the original campaign, you can try some of the fantastic mods available, especially Lilith: Battle for Jalavin (a totally newly created world which will take you up to 20 hours to complete). This mod in combination with other well known TQ mods such as Masteries mod (skill overhaul mod), gives more than enough incentive to go through the whole experience all over again with a fresh new slate.
All in all, if you are a fan of ARPG's it is a genuine crime not to have this masterpiece. The majority of the now-closed TQ studio (Iron Lore) are in the process of making a spiritual succesor named Grim Dawn (by Crate Entertainement). Given their history with TQ, this is a title to keep in mind as well and currently available through early acces on steam.
Closing advice: go spend money on this, preferably as soon as possible, you will not regret it.