If you look around all the manufacturers, Ultimate Ears tend to get more good reviews than Shure and most other alternatives (Westone, for example) come in more expensive.
I've just bought a pair of Studio 3s from
handheldaudio.co.uk for just over £50 inc. p&p. Unfortunately, the pair that came through appear to be faulty, but I'm sending them back for a replacement (I was having problems with the earphones hissing a lot, even when plugged into pro equipment).
When I was trying them, they were pretty good, provided what was going into them was good, low bit-rate mp3s suffer like anything. Detail retrieval is absolutely astounding, sometimes you hear things that some £3k hi-fi kit wouldn't show off. Soundstaging is also really good and whilst they don't have the bass response that some crave, the rolloff is pretty good. I'm keeping the PX100s for home though.
But this is indoors, i.e. quiet.
When you're on the go, and in a noisy place like a bus or a town centre, the extra detail makes the Studios a far more exciting listen than something like the PortaPros or the PX-100 (which is what these 3s are replacing for me), especially considering that the bass response of these tends to tail of and be masked a fair bit by low frequency ambient noises like vehicles rumbling.
Fit is also fantastic, really comfortable, and you couldn't get them to fall out if you tried.