You're quite right, the reputation is for "old and dirty". But the thing is, if you provide a reliable train with a nice interior, it'll seem new, regardless of how old it actually is. It's not as if people complain much about HSTs, despite them being 40 years old. They're reliable and their interiors are nice, with the one thing that lets them down being the doors.
As for limited usefulness, sure, their usefulness is limited, as is every other train's. Different units have different purposes - you wouldn't see a Voyager operating the Stourbridge Branch for instance. But what they do is provide a low cost, comfortable DMU to tide over until electrification becomes more widespread allowing new-build EMUs. Perhaps they're not ideal, but unless you really want to keep Pacers...