I can honestly say that the p-38J is new to me. I got the p-39F in my first week playing the game (its just such a classic and i loved it years ago in CFS3), and meh, I gravitated towards light fighters, and acquired the F7F through the mustang line. However recently I've come back and have fallen in love with the P-38F (and SE 100), and I must admit, I have fallen for the P-38J too. Mine isn't specialized, but it will be soon.
I liked the 23mm, but I'll add to the chorus, you've got to go with the 37mm it really is good. I complement it with all the ordnance I can cram on it, and its a blast. To complement that gun choice, I've gone with long gun barrels to extend the range of the guns, since the .50s and 37 are fairly short range (plus it doesn't hurt accuracy). My 2nd and 3rd skills are/will be the 2 marksman skills to further enhance accuracy and I will get a sight when specialized.
The thing is glorious, and the ordinance allows you to cap things easily. If you have even a little help you can take a plant since you can get it half way there with your own ordinance. Then set up to attack enemy intruders for more cap points or move on to the next objective. The speed an maneuverability mean that you can chase and kill other heavies fairly reliably. As a general rule however, unless you can be sure of a kill, have significant backup watching your six or the enemy is completely focused on another target, do not attack human fighters or even many multiroles unless you have a significant altitude advantage and can use a dive to build up speed, and have a reserve of boost to escape.
One great thing is its ability to climb rapidly, and its good ceiling. During the invasion game mode in particular I found that there were quite a few games where I was the only one on the team who could get up high enough to take out the enemy human high alt bombers.
Full disclosure, not the most experienced or best heavy pilot:

Oh, possibly the most important thing, it's gotta' look good: