I'm anxiously awaiting Aero Precision's M5E2 (Gen2) upper to be released. If it doesn't suit me I'll probably go with a Mega upper/lower combo. From there, probably a Krieger barrel, JP BCG, SLR adj block, etc...
Three days ago Aero told me they had 44 M5E's in stock. I would have bought one but I was not clear in my mind what rail/guard was congruent and they could not quickly explain. I got the impression, (and I could be wrong) that the M5E fit to handguard is proprietary. If anyone knows better please let me know.
I was altogether ignorant that the ones coming on line the end of this month were Gen2. The Aero website can be frustrating.