Season 15 (though I am only 2/3 rds of the way through it) seems to be widely and rightly regarded as one of the weaker seasons, at least from the time Williams takes over (Horror of Fang Rock seems to have a pretty good rep, again I think rightly). Opinions seems to divided over these episodes. I have seen several reviews of people who really loved Image of the Fendahl and those who hated it, and likewise The Sun Makers seems to have more than its fair share of detractors, as well as its admirers.
So, is it that bad? I don't think so -- it is certainly a damn sight better than The Invisible Enemy and Image of the Fendahl. Is it that good? No, I don't think it is that great either. It is a perfectly serviceable, middle-of-the-road sort of story deserving a middle-of-the-road 3/5.
I think one of the reasons people don't like it so much is the use of humour. The Collector and the Gatherer are pantomime-villains who ham it up with gusto. I rather like the performances, and like the fact that they are given a different motive than usual. Instead of taking over the universe, they just want to tax it! And I like the fact that nobody seems to really know why the Company (praise the Company!) is there or what it is doing. Even the Collector doesn't really seem to know ("We make profit! Take a look at our prospectus!"). On the minus side, I think some of the humour (and, curious to say, some of the violence) seems ill-judged, and things like the red button jumping out with a boing and the Collector disappearing figuratively down the toilet are aimed at children. I do love the Collector going round in circles in his chair though!
Some of the criticisms I have seen have missed the point I think. The corridors are boring -- yes, its meant to be a grim, featureless dystopia! The same goes for the character of Marn who is supposed to be as sexless and featureless as possible. She is just the kind of person that someone like the Gatherer would keep around. Mandrel could have been played better, I think, but I like the idea of rebels who are not especially heroic.
BTW, I think it is better than (similar in some ways) The Caves of Androzani. COA is one of those stories that I seem to have I blind-spot for. Nearly everyone seems to love it but I've always though it was pretty weak, even if it did make Peri my favourite companions