TQ's biggest drawbacks are the bugs in the questing that can make progression impossible paired up with the worst leveling system I have ever seen in a video game. The rest of the game is decently well done, although not amazing. At $5, it is worth a try. Read on if you dare for comparisons and such
TQ is a decent game, and the Gold is defnitely the way to go if you are wanting to play it. At $5 it is worth picking up, but don't fool yourself into thinking this is anywhere near the quality of a Diablo. The leveling system, although far more open than Torchlight's or many other games, is garbage. The atmosphere leaves a lot to want as well. That being said, I would still say it is worth trying out at $5 although I doubt I will ever get around to beating it even though I have owned it for a couple months now.
The quest system in TQ is terrible, as even in only my five hours or so of playing it I encountered many bugs. On the first quest alone, me and a friend had to create three new characters because the first quest in the game repeatedly bugged and the NPC refused to allow us to complete the quest. The only redeeming quality of TQ's quest system is the shear abundance of side quests.
If you want something to bide your time till Diablo 3 comes out, check out MeridianXL or some of the other D2 mods. They have a LOT more to offer, and they are free assuming you already own D2.
To compare TQ to Torchlight... I wouldn't say either one is greatly better overall. Having beat Torchlight over the holidays after picking it up on the sale, I can definitely say TQ is harder. Torchlight left me very sad that there was never a single difficult encounter, although some were a little tedious with the need for slow cautious play. Torchlight has far far better appeal visually to me, because I love the artistic style far more than attempted realism. TQ has a much more immersive world than TL. The terrain in any area will vary a bit with hills, cliffs, trees, caves, etc. making it feel much better than just bouncing from one rock to the next.
And finally, to compare to Sacred 2... Well the two had a lot in common. They both had very large very open worlds to explore. Sacred 2 is graphically far superior, although the engine in S2 is buggy and can cause many problems with freezing/crashing on some systems. Questing is more fun in TQ as chains actually have ends. Playing Sacred 2 if felt like a quest would never be over as they continually would make you bounce around an entire zone just to complete one quest, stopping in town five times just to tell the NPC 'Hi!' so they would give you permission to move on. It is much more straight forward in TQ, and the closest quest chains come to this are 'Thanks for saving our town, now please go meet the mayor of the next town'. I found the leveling system in Sacred 2, althogh akward, to be far superior to the garbage system in TQ.