They really weren't. Cheap and nasty was the verdict at the time - very Virgin brand. The impracticalities of the internal layout were immediately obvious. The trains managed the rare feat of having comparatively few seats per coach, yet simultaneously being very cramped and having virtually no useable luggage space. They're like a tardis in reverse.
Individual reading lights and window blinds were welcome additions. Innovations like the on-train audio entertainment were quickly dispensed with.
Mechanically, the trains seem to be OK. The amount of diesel power means they accelerate quickly compared to previous generations of diesel. But the claim that the trains would be as quiet as loco-hauled was quickly found not to be the case. The tilting function on the 221s was an expensive failure and abandoned.
Personally, I find them OK if not heavily loaded. Unfortunately that is rarely the case.
Also no train should smell of wee. That's a fairly basic requirement in my view.
However, provided the underlying mechanicals are still in good order, there's nothing that can't be solved by taking the interior back to the bare metal shell and doing a complete refurb.