Even older iPod's have excellent sound too. My personal favorite (of the iPods I own) is the 3rd gen iPod, for sound characteristics and quality. But that one connects and charges using FireWire, so it's harder to "manage" these days... 4th gen iPod sounds great too. And they both still works fine with the latest OS X and iTunes. The newer 5th gen iPod should not have any problems with the any supported version of software.
There are always "people" who have problems with anything, especially when there are millions of users out there. If just one-tenth of 1% (0.001) of a million uses had a problem, that's 1000. For these old iPods, it's usually because the iPods are becoming old and faulty (often with its delicate hard drive), not because there's a compatibility problem with the software. So seeing a few dozen (or even a few hundred) posts here about a particular problem does not mean you'll have that problem.
I've replaced the hard drives in my 3rd gen iPod, 4th gen iPod, and iPod mini with (flash-based) compact flash and SD cards, on an adapter. They work better now than when they were new.