Im a tight-wad by nature, I also have 4 kids demanding ever penny I have. I researched the best and lowest priced airsoft I could get. I called about 15 websites finding out what their most popular and least whined about returned airsoft rifle- repeatedly is was the JG/Echo G-36
The Echo G36 is a re-branded version of the JG G36.
The Echo's come with a warranty and a lower FPS.
The JG's come with no warranty and a Higher FPS
If you go with a JG or Echo try to get a version III release. this third version/release (not meaning type 3 gear box) has shims installed on the gearbox.
The G36 comes with a Type 3 gear box making it more reliable.
The plastic G36 is also a "clamshell" design making it inherently strong.
The plastic M4's are known for wobbly barrels and stress points- go metal with an M4.
The M4 does not use the more reliable Type 3 gear box.
I put six 10,000 round tubs through my $ 109.00 JG with no problems.
Reliability wise my JG out did all the high end stuff all the time- yet they raved at how awesome their gun was.
Regular G36
http://www.airsplat.com/Items/ERM-JG-G36C-138.htm
This one accepts M4 mags
http://www.airsplat.com/Items/ERM-JG-G36C-638.htm
Some of these JG's are shooting 400 FPS out of the box
http://www.airsoftgi.com/index.php?cPath=139_24_166
If you do go with a M4 style rifle try to get a "S-System or a 614 rifle. The top rails eliminate the barrel wobble.