I just got it a couple days ago and I'm pretty happy with it so far. I picked it up for $5 on Steam and for any that are interested you can still get it for $5 from Steam or http://www.torchlightgame.com/ .
First off, it is a Single Player only game. If you are only interested in online play for games like this, skip it.
I don't really mind the more light hearted graphics. The only presentation issue I have with it is just how many over the top explosions,magical glows, etc. there are in battle. At times the light show makes it hard to see what's going on. That actually has me a little concerned for D3 as well, though I don't think it'll be as bad in that.
The gameplay in torchlight is good although it has a few minor flaws(constantly having to refresh buffs, can't change the controls aside from your 1-0 hot keys. Don't want 'A' to be the autotab button? Too bad. One strong point is that gold is actually valuable unlike D2. The merchants sell great equipment and you can always pay to improve your current stuff(with an increasing risk of losing all properties). Being able to send your pet back to town to sell items is handy too so you don't have to travel back all the time.
Torchlight's story is junk unfortunately. It definitely feels like it was just slapped together as an afterthought, unlike the Diablo games.
As for replay, Torchlight does use almost all randomized dungeons which is a plus, but you wont need to restart with different builds over and over like in D2. With Torchlight, any skill is functional at level 1(out of 10) and you get a lot of skill points to distribute. As a result, you basically only need to play through with each class once to see all the skills and wont feel the need to replay with different builds. This is good in the sense that you can switch it up regularly while playing and don't get stuck spamming one skill for the whole game like in D2 but again, it isn't as great for replay value. Still, I expect to get more replay out of Torchlight than I did from Titan Quest, which lacked randomized environments, causing it to lose my interest after one play-through on one difficulty.
I would consider Torchlight to be a lot better than Titan Quest, but still a big step away from D2. You probably wont get the kind of replay from Torchlight that D2 offered.
Worth $5, oh yeah. Worth $20? I guess that depends how much you expect to be bothered by less realistic, more light hearted graphics and a lack of interesting plot. If you like Diablo-esque ARPGs then I think it's worth the $20.
And as others mentioned, it is kinda neat to hear some of the sounds that seem to be taken right out of D2:P