IMO the P-38 only seems so impressive, because other American early war fighters were rather unimpressive. The P-38 was very expensive and difficult to maintain...
I think twin engined fighters in general and this one in particular would always be the exception, not the rule.
Here's an excellent thread at a
WWII aircraft forum about the merits of the Lightning versus the more renowned Mustang.
There is no consensus over which is the better fighter (though the Merlin-engined P-51 is the more popular, naturally) but nobody believes the P-38 to be a turkey*.
It would seem that the main advantage the high performance Mustangs had over the later Lightning models is not as much to do with performance or manoeuvrability (though there was some advantage for the P-51 in the second category, even moreso with visibility) but in the areas of ease of production, and most importantly, user friendliness. In short the P-38 was a difficult plane to master, it was expensive, and it wasn't as numerous as the single-engined planes America built.
And that was the models produced in the last two years of the war. Between the plane entering service and the second half of '43 it had a huge number of bugs, partically in the area of controllability at high speed. I would hazard a guess that the Lightning's greatest defect was that it took so long to make into a first class fighter, whereas the high performance Mustang was taken from the stage of concept to being the dominant fighter over Europe in barely two years. Technological development was on the side of the P-51 Bs, Cs, and Ds.
Markus said:
If the single engine fighters hadn´t lacked range and high altitude performance -something the P-38 only had in a warm climate- this contraption would have probably phased out quickly.
In another thread I linked to the best performing Lightning ever proposed, and I have no doubt that the P-38K could have been a dominant air superiority fighter (and the Js and Ls were no slouches); of course if they had been put into service in the cold airs over Northern Europe too early they're likely to react badly to the poor quality British aviation fuel, thus negating the nominal superior performance. (And as for range, the Mustang and the later P-47 models were as good as the P-38. So the very fact it maintains some reputation is testament to it having other qualities.)
A dominant P-38, as unlikely as it was (the lessons of R&D and real-world experience has to be sped-up, dramatically so) could have seen the twin-boom idea viewed as a war winner.
*Except maybe some German posters. Oh, well.
