I previously had the Sennheiser MX450 and the AKG 14P for comparison. I think the 450 had better mids but I'll be comparing mostly to the AKG since I have used those longer. The 14P had (for the price) a fantastic amount of detail in the treble and mid region, and what bass it had was tight but didn't extend very low. I really warmed to the sound of the AKG and honestly the MX51 just don't really compare well. My first impression out of the box was that the treble was non-existent and the bass was boomy. I left them on overnight and then listened with them for a week so I think I have a pretty good impression of them now. They sound good (relatively) but I had to bump the treble all the way up on my DAP. It also helped to punch a whole in the foam covering the earbuds (a very small hole is enough).
I like that the MX51 has more bass quantity but it's still somewhat boomy and not as detailed as the 14P. The mids and treble also are not as good as the AKG, though I find the MX51 to be more balanced overall (edit: this is considering that treble was EQ'd all the way up). Another thing to mention is that the 14P has some soundstage whereas the MX51 has none. I will keep them around because they're smaller, more comfortable than the 14P and offer a nice change of pace. However, I will be trading up for maybe IEMs to use portably since I've now realized I can't live without the detail (blame/thank AKG).
Edit: Forgot to mention that the cables on these have some trouble with microphonics.

I find these earphones fit much more comfortably than the standard iPod ones - a little more snugly, too. The sound quality is quite good, definitely better than I was expecting given the price.

The case is more of a novelty than anything extremely useful, but who ever bothers rolling up earphones and putting them in a case anyway?

Overall a fantastic buy, I would definitely buy them again.

My iRiver last year came with headphones like these. I loved them!
Then the cord got cut. Called iRiver; they no longer have such, but told me to check for Sennheiser online products. So happy to have these "back" again... missed the deep solid sound so much!! I tried Sony, and ipod and everyone else while I searched.
These are the best. especially for under £12... not like that other more expensive ones I bought. Thanks to them and thanks to you. I will buy another now...just in case you stop selling them, but, please dont!