I haven't had the chance to see one in person yet, but CA is definitely trying to step their M4 game up. Rotary hop-up, mosfet, quick change spring.

Is there a brand of magazine to look for or avoid? Is there one that seems to be the most cross compatible with the different M4s out here? From searching the forums, the concensus seems to be low caps?

From what I've heard, PMags and Dytac are the way to go if you want awesome feeding. In my experience, low caps feed just as well as a good mid cap, but a mid cap will hold 2x or more ammo so mid caps are better in general unless you really wanna simulate the real stuff.

PTS mags are probably one of the best mags you can get, they have a nice window so you can see when you've fully topped your mag up and they feed like a dream.

Guns I used them in:
VFC HK416
G&G SRL
Magpul Masada PTS