In general it seems that the film succeeds or fails based largely on how much you enjoy the chemistry of the two leads.
Spenser Confidential is sitting with a 47% on Rotten Tomatoes, which isn’t the worst score.
It means that about half the reviews are actually positive, though, it should be said that few of them are particularity glowing.
The film is based on a book series after all, so the idea of a franchise of these things has to be on the table, and the movie does apparently set up a sequel, a few reviewers seem to be up for that, such as ComicBookMovie.com.