I think Shatner is right on this one. IMHO it doesn't matter if its his opinion, he's simply stating something which has become obvious to the "old" guard fandom. We're old geeks (well apart from me, I'm only 21 but I've watched every last TOS episode), and they don't like what Trek has become.
TOS, TNG and DS9 were awesome series which really boosted Trek IMO. VOY and ENT were in a category I won't define. I watched most of the VOY episodes and I watch ENT if there is nothing else on TV.
And anyway, he's Captain Kirk, he's been typecast his entire life after that role. He can be a little bitter.
"The misery of being exploited by capitalists is nothing compared to the misery of not being exploited at all."
-Joan Robinson, economist