They're like anything else. Some people will love them, some will hate them, most will be in the middle.
When properly set up and driven, they can sound absolutely fantastic. The best system I've ever heard was the old Naim DBL actively driven by 3xNAP500 amps.
I've also heard Naim speakers, notably the old SBL, sound awful because they either weren't set up properly, not driven properly, or both.
One of my favorite systems was when I had a Naim 250 driving ProAc Response 2.5s. That combination sounded amazing - I just wish I had the space for something like that now!
As to worth, only the prospective buyer can answer that - it's far too subjective a question.