By most accounts it was excellent overall, but its effectiveness was theater dependent and dependent on role.

The P-38 was extremely effective in the Pacific Theater. It finished the war with more air to air kills than any other Allied fighter. Its dual engines, speed, and range were ideal for operating over the vast distances in the Pacific. It's powerful armament clustered around the aircraft's nose in a centerline position made it a very good gun platform, both for shooting down enemy aircraft and for strafing.

Where it wasn't as effective was as a fighter in the European Theater of Operations and the Med. The P-38 had some problems with early models not operating as effectively at higher altitudes that were more common in the fighting over Western Europe. It also had a slower roll rate than the excellent single engine fighters fielded by the Luftwaffe, such as the BF-109 and FW-190. A lack of maneuverability compared to single engine fighters was an issue that all twin engine designs faced. I would argue that the P-38 and maybe the Mosquito overcame this design issue better than any of the many other twin engine designs of the era.

Some of the shortcomings of the P-38 could be chalked up to tactical failings in the use of the plane, as well as the inexperience of American pilots flying in combat against veteran Luftwaffe pilots, as much as design. It was the only plane for a time capable of providing long range bomber escort before the P-51 came online, which arguably did not play to teh strengths of the P-38. The P-38 did became more effective in the ETO with improvements to the design and the tactics over time. It was the plane of choice for many of America's top aces. But even so, I don't think it would be considered a top air to air fighter in Europe the way it was in the Pacific.

The P-38 also made an excellent fighter bomber. The powerful twin engines gave it the ability to carry a large bomb load for a fighter, and its standard armament was great for strafing soft targets on the ground. Two engines also of course increased its survivability when flying into flack or performing other dangerous missions.

Lightening Strikes: The Lockheed P-38, by Steve Blake, is kind of considered the bible on the design and development of the P-38. It's a great read if you want to really learn about the plane.

All things considered, the P-38 was a pretty effective fighter that was also well rounded in terms of its operational capabilities.

The design is indeed a strange one, but it provided several benefits. The first is that the twin engine design allowed for a heavy payload located entirely in the nose of the plane. This meant that pilots were more capable of accurately hitting targets further away because they did not have to account for the point of convergence as the pilot of a fighter with wing mounted guns, such as the P-51, would have. The twin engines also provided other benefits. The P-38 was able to climb to high altitudes very quickly and fly at high speeds. Having two engines also mitigated risk to the pilot in case of engine failure/damage, and increased the odds of the plane returning home. Lastly, the size and shape of the aircraft allowed it to carry more fuel, which made it well suited for long range escort and interceptor duties.

For the most part, the P-38 was used effectively. There is some debate about exactly how effective the P-38 was, though some would argue that flaws in the P-38's service record are more attributable to misuse and poor tactics than the aircraft itself. These flaws were more apparent in the Mediterranean and Western European theatres, where the P-38s were also introduced before allied air superiority and were used with ineffectual bomber escort tactics that minimized the ability of the pilots to fly the P-38 to its strengths. However, top German aces often praised the P-38's speed, firepower, and maneuverability.

The P-38s were most popular and used most successfully in the pacific, an area for which they were well suited particularly because of their long range and high altitude, which made them ideal for escorting long distances bombing raids over the ocean. P-38's were credited with more kills than any other USAAF plane in the theater, and were flown by America's top aces, such as Richard Bong. P-38's were famously used in the mission to intercept and shoot down Admiral Yamamoto.

As for why the design wasn't replicated- it sort of was during the war with the P-61 Black widow night fighter. However, by the time the war ended, the era of jets had begun, and the P-38, along with most other propeller driven planes of the war, became obsolete.