Questions about KWA MP9

[Flanbix]

I am more and more interested to buy a MP9 as I love playing with pistols but I'd like to have a SMG.
But I have a few questions, first and usual: Is it any good?
Also I have seen that these is 48 round mag, 50 round mag or 55 round mag so I don't know which one normal fit the MP9?
Does the rail have problem, as I read somewhere that the pitch was 21mm instead of 20...

Thanks.