[THe_Silencer]

As a fellow P90 user I'll add in my 2 cents on my Echo1 P90:

Internals
Internally they're shit TBH. My piston stripped less than 300 rounds in, the piston head cracked, the bevel gear chipped, the pinion gear chipped, the hopup does not maintain its setting, and the brass bushings are overly soft. The tappet plate will mostly likely break if the gearbox is opened when the spring is under pressure, so beware. The 9.6v battery that came with it died but that's not really a fault of the gun. As well, the motor height is adjusted on the back of the motor cage via a hex wrench. Chinese p90 gearboxes are made out of softish metal and the motor cage will break if overadjusted. Replace with a TM motor cage if possible. The fire selector cutoff is prone to wearing out on all models.

As for good things, the stock sector and spur gear have yet to fail me, even under extreme (35+ rps) conditions. The wiring assembly is located on the outside so it's a cinch to do stuff like install MOSFETS. Due to the design, the P90 has the shortest length inner barrel of all the bullpup guns, about 247mm, or the same length as a G36Cs.

Magazines
Due to the mag design, reloading is cumbersome and slow compared to an mp5 or m4. This can be a turnoff for some. Speaking of mags, avoid MAG brand magazines, especially if you have a Chinese P90. I bought 4 and only one feeds well on semi auto. Apparently some people who run CA and TM P90s have no issues with them but YMWV. I have no issues with Silverback or Pro-arms mags but hicaps are crap for me.

Conclusion
As someone who's 5'11, I have no issues with holding it, I'm perfectly comfortable. As well, mags are not conducive to speed loading so this may be a major drawback in a CQB environment. If you're planning on getting a Chinese P90 over a Classic Army or Tokyo Marui, then be prepared to spend $150+ on getting the internals up to par.