I just started playing Trundle this week and I have to say I really like him. I literally NEVER see anyone play him and I like being a special little snowflake. On top of that, I think he's actually an alright champ and not terribly OP or UP.
I play him as a jungler and I really think he's pretty strong at this. He can clear both his jungle and the enemy's jungle without having to B and that's just great. With a little bit of practise, he can also gank pretty well. Contaminated Soil allows him to come flying out of nowhere and then Pillar of Filth (if placed properly which can fail often) can just totally screw the enemy's escape.
The only thing that bothers me about him is that he really doesn't seem to scale very well. Rabid Bite, which basically allows you to double attack (but wastes an auto attack if not timed right) doesn't seem to be as strong once you've got your W levelled to 5 and have zeal/triforce, because you can't really time your bites as reliably.
Also, Trundles ult seems kind of meh to me later in the game. It's a pretty deadly tool early and mid game, where a lot of the time it assures you a 1v1 kill, but later on in a team fight I'm not so certain of its value. Ideally you'd use it on a tank to steal as much resistance as possible, but I see a few problems with this. First, you're still not going to target the enemy tank in a teamfight, regardless of whether or not he's 25% less resistant, unless he's caught WAY out of position. Second, Trundle's not exactly priority target himself, so the resistance isn't a huge freaking deal. I guess if both teams lose their carries, and it ends up being a sustained slugfest, maybe then Trundle can help stay on top, but I don't see it being a big deciding move like a lot of people's ults are. Last, by level 18 with some decent gear, the life stolen from R is just negligible.