About two years ago I got the Shure SE210. It was good for some time, until I grew unsatisfied with the sound. Mids and highs were good, but the sound was severely lacking in the bass department. This summer I upgraded to the Shure SE530. Of course, it's quite expensive, but: wow! It's just awesome.
If you're looking for good sound from IEMs, it might be worth it to shoot for the high end at once. Buying a lower end IEM only to find it's not your cup of tea and then gradually moving toward the higher end will eventually turn out more expensive than going high end immediately.