Nemesis had a lot of flaws, but overall I found it to be enjoyable in a general "spend a couple of hours with the TNG crew one last time" kinda way.
The movie got a very bad rap online, I believe, because its script was widely available for download more than a year before the movie opened. Critics had a lot of time to tear it apart, which they did (I believe AICN and/or Dark Horizons had script reviews that completely blasted it), and created a lot of negative buzz. In addition, IIRC it was made during the first or second season of Enterprise, at the height of the "Berman/Braga raped my childhood" hysteria among (many) Trek diehards so a lot of people were just waiting to tear into it regardless of its quality.