Back to the OP's question, it depends for whom.
If you ask Hitachi, they must be overjoyed at how many orders they have received (over 320 AT300s will exist within the next 2 years)
I find they look and feel like a hospital clinic - cold ambiance, over lit and uncomfortable. I speak for most people (enthusiasts, insiders, normal passengers) However they are achieving very decent MTIN figures and will continue to, and they are cheap.
ToCs clearly feel they are a success because they are cheap and reliable and the only viable intercity option. That said, I'm not sure why no one has ordered that 125mph bi-mode Aventra, or more class 397s, 745s or class 68 LHCS which are all far superior IC offerings. These are also competitive options.
In my view, the program is a failure for the UK rail industry because the point of privatization is to encourage a competition for the best rolling stock at the best price. The AT300s are, to the top brass that are placing these orders, unrivaled in competition on all fronts (price, regulatory approval), so what we have is a sub-par train dominating the IC segment.