My feelings on Pokebank are mixed. I paid for it because I needed the extra space (and because I wanted to continue to transfer Pokémon from 5th gen), and I do use it frequently, but I've noticed that I avoid taking anything I really care about off of my cartridge. While I have multiple games (both versions, plus extra downloadable versions from the promotion last month that I haven't started yet), I don't really want to spend lots of time trading Pokémon from one game to another, nor do I want to have to release all of the trade fodder that I catch to do so. In that regard, I value Bank because it saves time.
One of the reasons I've started using Pokebank a lot is that I really hate releasing Pokémon. Thus, I just store lots of breeding rejects in Pokebank, trading some of them of when I have spare time. Ultimately, I'm just postponing the inevitable moment when I'll have to go through and manually release lots of them, and since Bank doesn't provide that much storage space, I won't be able to put that day off forever. But for the time being, the extra space in Bank allows me to continue my nightly ritual of breeding a couple of batches of Pokémon before I fall asleep without forcing me to deal with logistical hassles.
So, ultimately, no, I guess I don't really trust Bank. I've been pleased to see that, since its [re]release, it's been relatively stable, and I hope that the fact that it's a free service will encourage GameFreak/Nintendo to ensure that the servers run smoothly in the future (especially after the disastrous situation in December). If you need the space or the convenience/time, I would use Bank's storage, but only deposit Pokémon that you wouldn't be heartbroken to lose. About half of the Pokémon I have in Bank are ones I really ought to release/trade away at some point, but I keep thinking they might be valuable, so I leave them there; the other half might be irritating to replace, but wouldn't represent a major setback if lost.