Very very good. Right out of the box all mine would run at a crawl at #1 on my DT400 throttle. I didn’t need to change any CV’s to improve on it.
With Proto 2000, there is a lot of variability. You really can’t make a general statement and cover them all accurately. As a rule, they are pretty good, and as you go from older to newer/more recent, they get better and better as a rule.
The cracked gears are yes, a problem on the older late 90’s models, but easily corrected by getting replacements which are often replaced by the maker (Life Like, and now Walthers) or cheap if you have to buy them. You just remove the gear box cover and replace.
The oldest Proto 2000 had good looking shells on essentially cloned Athearn blue box chassis, which were more reliable and better QAQC than Athearns own chassis. The down side is they stll had the crappy coupler mounts, but the up side is they generally ran smoother and quieter than the stock Athearn drives.
As you moved into the latter 1990’s, Proto improved the chassis and the shells got more detail. I have 11 of the GP30’s and they were nicely done. I bought an E8 and the printing was smeared on the nose and the nose/windshields were not very well proportioned or executed. They were worse than the 1980’s tooling on Stewarts, a good deal worse. I returned my E8 and never bought anymore. The SD60 wide cabs weren’t done very well, had a wide gap in the cab vs body. The GP60’s were nicely done, got one of those. The newer Proto 2000 diesels are either newly tooled and better, or upgraded older Life Like models and pretty nice, but the price is not so nice! The recent P2K seems to be on par with the newer Genesis and Atlas models, give or take.