I've bee through 4 pairs of these. They sound good enough for what you pay for them, the problem I had with them was that, over time and continued use, the wires would sever just before the 1/8" plug. I stopped buying these.
They're ok. If you're an audiophile then you're not going to be blown away (no surprises there), but if you're looking for something cheap and cheerful for day to day use, won't worry about losing, exersice, listening in the rain etc, I imagine they're hard to beat for 20 quid.
That said, if you're coming from JVC or Apple's earbuds, then these will be a very pleasant change!
P.s. there is little to no difference between the M6 and S6 in terms of sound, the S6 is just reportedly more water/sweatproof.
MEElectronics Sport Fi M6 In-Ear Monitors Reviews (3 total)