I like my M10's a lot and for two channel, small rig and
I have no desire for anything else. Some people say they don't like the gain level wheel and feel it can get accidentally changed too easily, but that's not a problem I've had. You might find something a little bit cheaper that would do about the same, but the M10 has been so reliable for so many people it's hard to see a reason to change unless you just want to try something different.
There are times I'd like to have a more secure connection from pre to recorder like a locking trs mini jack or XLR or something but a right angle plug and a small piece of gaff tape does pretty well and keeps form factor small.
Personally I'd go for upgrading mics and pres before feeling I needed to upgrade my m10's. I guess if I was going to do it I'd look closely at a used Sony D50 or the successor, the new D100 but it's a lot more expensive than an m10 and a bit bigger.
Of all my gear, my M10 and R-44 give me the most sense of comfort regarding reliability and performance (followed closely by my tinyboxes). I have gear lust at times to try new stuff and add channels, but can't complain about these guys at all.