Pros: Cheap; great overall sound quality for the price; tiny and lightweight with decent fit; suitable for physical activity; large selection of tips
Cons: Painful 6k spike; single-flange tips are bullet shaped
Unlike many people I find the fit very good. These are the first IEMs I've owned with memory wire, so for the first few days I had trouble figuring out which side was left and which was right without looking at the (barely visible) markings on the housing. Once I got used to it they were very easy to put it. I use three fingers - I hold the housing with my thumb and index finger, and use my middle finger to push the memory wire over my ear. Push inwards, twist backwards, and it's in. Very little microphonics, and won't fall out during any type of physical activity unlike most universal IEMs.
Bass is definitely emphasized in a typical consumer-oriented fashion but not overwhelming and doesn't intrude too much into the mids. Mid-bass is balanced out with sub-bass which is nice. Too many consumer-oriented headphones end up a boomy, muddy, mid-bassy mess with no real extension into the sub-bass (cough*Beats*cough). These extend very well and still strong past 25Hz. It's not heavily textured like higher-end sets but still very musical - definitely not one-note bass.
Mids are clear and surprisingly linear for a budget set. Resolution and clarity aren't on par with my $100+ headphones or even the JVC FX31, but nothing to be ashamed of, especially at what these cost. Mids are on the lush, warm side of things but not overly thick.
My biggest problem with the M6 is a huge spike centred at 6k that makes the treble painfully harsh and bright. Mind you, this is with the tips I use, and with my ears. Everyone's ears hear differently so you may or may not have this problem. Using a free-form equalizer I was able to fix this, and otherwise the treble is very good. It doesn't have much sparkle but is extended and smooth.
Edit for clarification: in order to EQ out the 6k spike you will need a free-form equalizer such as the iOS "EQu" app. A narrow 4-6dB drop centred at 6k (back to neutral at 4k and 8k) will do the trick. A standard 10-band equalizer will likely not work as they typically only have bands at 4k and 8k, which would drop the whole 2k-16k range, resulting in a very dark sound with no treble. With the EQ fix, these earphones sound very decent against other budget-fi sets.