There's an outdoor museum of battle trophies -- artillery & armor -- at the Army's Rock Island Arsenal in Illinois which I visited a few years ago.
The Russian-made pieces captured from Iraq in the gulf looked like scrap iron -- all rusty, poor fit & finish, ugh. There was one American made gun (vintage 50's or 60's) that had been sold to the Iranians under the Shah, captured by Iraq in the Iran-Iraq war, and then captured again by the Americans in the gulf. It looked far better than the Russian pieces, but still a bit shabby.
The German materiel captured in WW2 looked like it had been built by Mercedes-Benz (as some of it probably was), and absolutely looked like it could be given a shot of oil and some repairs of (obvious) battle damage ... and sent right back into combat. Actually, the Germans' awesome craftsmanship was part of their problem; they couldn't make those beautiful implements fast enough to replace their losses.
While beautifully crafted they suffered because they were mechanically too complex which invited breakdowns under combat conditions. This was exacerbated by the too quick rushes to production without proper field testing to work out the bugs, and a lack of trained mechanics in the units employing them. Often, even a minor mechanical malfunction could render them inop until someone could figure out how to fix them.
While retreating during the Normandy break-out and the rush to escape the falaise pocket, the Germans abadoned many perfectly good tanks that stopped running due to minor breakdowns. A fair number of these tanks were recovered, fixed by U.S. armored units, and sent back into battle with a fresh coat of of OD paint and big white star on it.
On the Russian front the Germans and Russians frequently found themselve fighting their own tanks which had been recovered by the enemy. The same thing happened, though not as widely, on the Western front. You hear almost nothing of US tankers riding around in captured Panzers because it was felt that it would upset the folks at home (bad PR) who were working night and day to supply GI's with "The best equipment in the world." I have seen pics of Mark IV's and V's as well as Tiger I's in American war paint alongside Shermans.
The main reason more weren't used was because of Ammunition. The supply was plentiful (we captured tons of it), but unreliable. Widespread sabotage perpetrated by the slave labor the Germans used in the ammunition factories made relying on captured German weapons a gamble and the practice was discouraged.