Honestly, I don't mind Nemesis as much as a lot of people, but it's definitely far from the upper tier of Trek films, or even the Next Gen films. My biggest problems with it generally revolve around bits of the direction, mostly flat attempts at humour (I rank Insurrection even lower than Nemesis, but at least that one hit the mark more often than it missed), and Shinzon's motivations against Earth being somewhat less than believeable.
Otherwise, it's watchable stuff. You could even say that it accomplishes a lot of important things, character milestones mostly, which is more than Insurrection managed to do. But Nemesis also missed a lot of opportunities, too. Much like the entirety of Voyager before it. But that's another rant.