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.
Don't get me wrong, much of the online criticism prior to the movie's release was justified, but nonetheless had many fans primed to hate the movie long before they saw it.
I remember seeing it opening night by myself (my wife, who usually is a good sport about indulging my geekness) took a pass because she thought the trailer looked "ridiculous." I was in a nearly-empty theater with maybe 10 other people, all of whom, like me, looked somewhat embarrased to be there. When it ended, there was dead silence and we all slunk out, kinda hoping no one would see what theater we came from.
The contrast a few years later, when I saw Star Trek '09, was astonishing. I saw that film in a packed house, the audience laughed at the jokes and cheered along with Kirk, Spock & the rest during their adventure, and applauded when the movie was over. It reminded me very much of the audience reaction when I saw Wrath of Khan in the theater way back in the early '80s when I was about 12.
Both flawed movies, but audiences loved one and hated the other.