Downloaded it, played it for a while. It's a mixed bag. I like the new class - rather than having a few good skills and a bunch of crappy ones, there are a ton that seem good, and it's tough to decide where to put my points. A few of them actually seem a little too good. There's a passive skill that at level 1 heals you for 20 points/second for every monster within 10 meters, plus does a few other things. Since you can only get hit by so many people, this pretty much means you should go get mobbed, and you probably can't die.
There seem to be a lot of monsters, and even from the start, you find elites, which drop great loot and beef up your fame XP, which isn't necessarily a good thing for balance. They drop a lot more gems than before, which is kind of nice, but they're not any more powerful than before, which is probably also good.
And they've done stuff to the inventories. They've definitely increased the shared stash immensely, and I'm not sure yet, but I think they've added several pages to what you can carry on your person.
So what I'd say is that it's fun, but you may not love everything about it. Definitely back up your saves and the original PAK files before playing, as these instructions suggest. http://www.t2mods.com/how-to-install-torchlight-2-mod/
Oh, and if you can't find where Steam installed Torchlight 2, it might be at C:\Program Files (x86)\Steam\steamapps\common\Torchlight II (I'm on Windows 7 64 bit). While I was in there, I noticed that there's a soundtrack folder! Not the music files the game uses, but a set of mp3s and a pdf CD booklet. Very cool - kind of bummed I didn't know about this before.