I've been commuting (Oxford or Didcot to London) with FGW for the last 18 months or so, usually once a week, along with other leisure journeys - and I have to say, in terms of customer service, FGW are really good. Friendly, knowledgeable, helpful staff seem very much to be the norm, both on trains and on stations.
As for what's not so good....well. Turbos for one thing. That is a definite curse on Thames Valley services! (Yeah, they aren't Pacers, sure, but still...) And the consequences of having to run a complex set of services, using trains that are not the newest, on lines that are subject to extensive upgrading...which does lead to delays.
I'm not sure how much it is a question of image: to my mind First's train services (even, he says grudgingly, FCC) have an overall far smarter and professional presentation than their buses generally do (and frankly First Buses, overall....are very much the poor relation of Stagecoach and Go-Ahead, certainly, and probably of Arriva too). To some extent FGW have to make the best of a lot of difficult circumstances, most of which are beyond their control (but still: owing a few HSTs of their own, and upseating the HSTs, - that was a good initiative on their part, and one worthy of praise. Bringing back the 180s, ditto): but, I think I would still say, for the most part, they make the best of difficult circumstances, and generally do a pretty good job in so doing.