I used to buy Koss Plugs, which stay in your ears very well and sound fine. But the last couple of pairs I bought were useless. One lasted just over a week before one side died without any major trauma (that I can remember anyway).
The first pair of UE200's I bought have so far lasted over two months, which might not sound a lot but I wear them for at least 30 hours a week at work. So I'm very pleased.
These don't stay in quite so well as the Koss Plugs, but they're not far off, and come out nowhere near often enough to be annoying (unlike many other sets I've tried over the years). And sound quality isn't a problem either. I have no complaints... especially as I only paid 5 pounds in a recent sale. They're easily worth much more than that, but I'm always a bit reluctant to pay too much because earplugs have a nasty habit of dying on you just when you think they're doing well.
I just hope I haven't tempted fate by writing this review. :-)