The CA P90 is a great CQB weapon, but recognize that the Hi-Cap mags have the least dependability compared to any other gun. I find they're not good enough to use as anything other than a backup mag. The box-mag style is kinda nice since it uses M16 hi-cap mag input, but I don't use one since I switch hands constantly whenever I play. The box mag requires you to shoot right-handed only.
This is an example
The P90 is great for close quarters, since the short muzzle. Less likeliness you'll break your barrel in tight quarters. The gearbox is also one of the easiest to extract. When the retractor plate is removed, it literally falls out of the gun!
I have the CA P90 Sportsline series, and I've found the all magazines besides Tokyo Marui don't "click" in. What I mean is the CA, Echo1 and KS magazines fit in the gun, but hold the gun to the side and the magazine will pop out. The TM ones are the only ones I've found that actually lock in. Too bad they're about $30 for a midcap.
It might be fixable with some filing.