FWIW, I picked up an G&G Combat Machine M4-A1 w/ blowback (I know not everyone's a fan of the mechanical blowback, but I wanted it!) for about $150; the non-blowback goes for a little bit less. It handled pretty much perfectly out of the box; once I got on the field, I just had to mess with the hop up a little and it was shooting straight and accurate, no problems, and with good distance. FPS was good enough; with the adjusted the hop-up, my shots were going where I wanted them to. Using the included hicap mag or new midcaps, I had no jams or feed problems.
The body isn't metal, but doesn't feel like plasticky crap and can apparently take quite a beating. If desired, G&G does offer an after-market metal upper/lower receiver, or I think they sell a metal version of the M4/M16. A lot of people on the internets complained about magazine wobble, but I didn't have any problems with it. The delta ring is a little stiff at first, but that's good right? Your handguard won't come off! I think it comes stock with a tightbore inner barrel too (6.04? 6.05?). Charging handle opens up access to the hopup, but the bolt catch is nonfunctional on this particular model.
From what I've heard, compatibility with parts isn't bad at all; I have a friend who has the same rifle but with a RIS, crane stock (you'd have to use one or a PEQ box if you do anything but a handguard, since that's where the battery goes), and one piece barrel, with no reported problems of having to fiddle with getting anything to fit.
My three complaints:
(1) The grip/trigger guard assembly: they don't sit flush where the parts meet, so without gloves it starts to bite into your hand a little
(2) the dust cover: pops open if I so much as glance at it. For some odd reason, my friend's refuses to open without some manual effort (it's supposed to open when the rifle fires due to the blowback mechanism, or when the charging handle is pulled).
(3) Barrel assembly: the outer barrel, front sight base, and orange safety tip are molded (glued?) together. If you want to swap these things out, you pretty much need new all of these.
I agonized for about a month and a half about what rifle to get, bouncing between the ARES G36K and the G&G M14. After looking into aftermarket parts, availability, and cost, I realized that these options cost way too much for how new I was to the sport. It was worth it to get an affordable rifle, with good compatibility and decent operation. I'm pretty happy with my purchase.