No regrets, here. If it was all fun, all the time, I'd have to stand in line and pay admission, but it can be very satisfying.
I think it's almost inevitable to have moments of regret, or at least doubt, during school and the first year of practice. You'll have to decide for yourself about the whole "nurses eat their young" debate. It hasn't been my experience, but even in a very supportive environment, it's a tough transition. If you don't occassionally feel like giving up, you probably aren't getting the full experience--or maybe you just have lot's of self-confidence. (Then again, I seem to recall that I had lots of self-confidence, before nursing!)
As is probably true of most endeavors, the feeling of accomplishment is proportional to the difficulty of the challenge. I have, on occasion, left the floor wondering what there was in this world that I couldn't do, if I set my mind to it. For me, at least, that outweighs the few times I've left the floor wondering whether I was really adequate for this job.
In between those extremes, there are lots of times when you might not be making life-or-death decisions--many patients will get better as long as nobody kills them--but you've helped someone be more comfortable and possibly recover more quickly, or headed off a problem before it became life-threatening.
Plus, I look damned good in scrubs