Sorry to go against the popular vote here, but I've had my Colt/Umarex M4 for about three years now and it's been flawless.
I initially chose it over the M&P because it wasn't plastic and it had the weight and feel of the "real" M4.
It's been 100% reliable with CCI MiniMags.
I haven't experimented with bulk ammo, and I most likely wont.
I tend to stick with what I know works well.
The "AR" platform allowed me to add a Magpul stock, FAB Defense grip, Magpul MBUS rear sight, and an ACOG replica scope. BTW, I have both 10-round and 30-round magazines. The 10-round magazines come in handy for shooting from a bench rest.
Here's a 10-shot group at 30 yards from a bench rest.