I've only been playing the game for since late November, so what I have to say might not carry much weight...
The way I view the P38 isn't as a plane that is great at anything. It is good at everything, however, and is able to exploit any weakness that other planes may have. You also usually have a lot of options on how to fight other planes, because it does so many things so well.
The only serious negative attribute the P38 has, in my opinion, is that pilot wounds seem to happen a lot more while in a P38 compared to other planes.