My first airsoft firearm was WE M4 Open Bolt GBBR (bolt really means chamber, I think).
I have really enjoyed it and I am very happy with my purchase and this is what I can tell you about it.
-It has some but not much recoil while AEG's have none, that is a plus since I am used to shooting real steel (firearms) and do enjoy the bang and recoil of real steel.
-It requires more maintenance than AEGs, that again is a plus since it makes it seem more realistic and still easier to maintain than real steel.
-Magazines for it will hold what the real magazines would hold (30 rounds) and are expensive to buy. AEG mag is about $12-$15 while GBBR mags are $50 and above, based on what I have seen.
-Had to get a gun doc to install a NPAS on it to meet the muzzle velocity limits of playing fields, it cost around $28 and took a week.
-If willing to pay for it a gun doc can instal steel internals and make it that much more durable.
-The only problem I have had with it is that lower part of the rail system (on the barrel) keeps falling off. It seems the delta ring is supposed to cover the rail system preventing it from falling off (I have never owned an AR platform so Im not sure how true that is) but the delta ring is too thick and has trouble fitting over the rail system. Of course there are solutions for this such as electric tape, plastic zip ties or the best option sand it to fit.
A Canadian is someone who knows how to make love in a canoe - Pierre Berton