I picked the P-38J to fly for a month or so. For me, it was a difficult plane to get proficient in for a few reasons. First, the J does seem to handle poorly at high speed, especially with roll. Second, I, too, was not used to a concentrated stream of bullets as opposed to a spread pattern of them. I would miss a lot of shots. Third, I would compress a lot. I'd dive down on someone or something, fixate on my target, disregard the groaning/shaking warning for a second or two too long, and auger.
Compared to most US planes, it climbs great. It also has good lethality and ammo load. I would sometimes not even use the cannon against fighters -- just the MG's -- if I felt I could get in a good burst. That seemed plenty lethal enough to put down a fighter. I'd save the cannon for bombers, for when I had only a snapshot, or for when my MG's ran out (which was infrequent). There are a larger number of planes that can both catch the P-38 and outturn it, which makes it more dangerous to fly than, say, a P-51, which can outrun most things that can outturn it, or a Spit, which can outturn most things that can catch it.
Once I got used to being careful of compressibility and improved my gunnery, I did OK in it and do like the plane. But, I do think it's one of the more difficult planes to get proficient in -- at least that's how it was for me.
