I bought one of these for my 3-year-old toddler last fall for about $50 used on Craig's list. She jumps in it every day. It's almost more of a playpen for her than a trampoline, though, as she keeps pillows and a sleeping bag and some plush toys in there.
It's been a great way to get her some activity and exercise on bad-weather days.
NewEgg has replacement springs for about $2 each. Mat and netting have held up pretty well, even though the toddler lets pets in there pretty often (notably, a great dane and a 20 lbs cat).
It has a 100 lbs weight limit, but I've been in there with her a lot (I'm 180 lbs), and my wife (at 240 lbs) has spent time in there, too, with our daughter (at 40 lbs). My wife and I don't jump in there, though -- just sit on the mat with our girl and play with toys.
Overall, it's been a good value. I bought it used, unsure if she'd really use it, but now that I know, I'd gladly pay $200 or $300 for one. Of course... don't have to.