Wow -- lots of nonsense being spouted about the P-38 here (like "relegated to being a bomber" total falsehood, as if 100 planes out of 10,000 determines anything).
I assume that people who have read a lot about P-38's have covered all of this already, but just in case:
The P-38 wasn't moved out of the ETO because it was a bad fighter but because of several other factors. One was that there were folks (including some with the ability to determine what was used) who thought that the P-51 was a better choice. (Note that "P-51 is better" is not the same as "P-38 is bad" -- both can be good, but one can still be better.) One was because the commanders in North Africa and MTO were clamoring for more P-38's specifically as a critical element to victory, and for more than were able to be supplied solely by new production of P-38's. One might have been as simple as inadequate cockpit heating in P-38's, which was sometimes a problem at high alts in the ETO.
The P-38 had a very long range, was an extremely sturdy plane (far sturdier than a P-51, regardless of statements here to the contrary), was used extensively in multiple theaters, acquitted itself decently vs. LW and Japanese fighters, and was excellent in ground attack. Notable achievements: first US fighter sweep over Berlin, shoot down of Yamamoto, top two leading US aces.
I've talked to many WWII pilots (several of them aces) who flew the P-38 (some in PTO, such as George Chandler; and some in ETO such as Bill Allen:
http://www.youtube.com/watch?v=5J0lEHyKInw ; some who flew the P-38 and the P-51; some who flew the P-38, P-47, and P-51; one who flew P-38 recon in the PTO, including flights at 40k). Of the ones who flew the P-38 and P-51, most liked the P-51 better for the role it was mainly used for, but every one of them thought that the P-38 was a great airplane as well. I don't remember one of them saying that they thought the P-38 was bad and were glad to get out of it into the P-47 or P-51, and they were always asked what they thought of the P-38 vs. the other aircraft they flew.
Good books on the P-38:
The Lockheed P-38 Lightning, by Bodie
America's Hundred Thousand, by Dean (chapter on the P-38)
Top Guns, edited by Joe Foss and Matthew Brennan (chapter written by John Lowell, who flew P-38's and P-51's in the ETO)
Fork-Tailed Devil, by Caiden (I've seen people crab about it, but I liked it)
Fighter Pilot, by Olds
