Slither is a genre-fan specific movie. I believe that the general population would not enjoy this movie, nor would they consider it a “good movie.” But I think I may be outdated in saying that. There has been a trend in films since Halloween of 2004 that leads me to believe in the wider acceptance of even sub-sub-genre specific horror films; that trend is to release a new horror flick every single weekend. Most of them are terrible; some are okay; even fewer are great. What this accomplishes is a larger exposure of horror films to the general public and a more viable market for them to be made. Higher volume does not always yield higher quality, but if the ratio of bad:good horror films is the same, this means more well-made horror films.
This is generally speaking. Slither does not fit into the catch-all, gore-fest horror film. As with a movie like Hostel, which was specifically created for fans of the exploitation sub-genre, Slither was created for fans of the B-horror-comedy (see Evil Dead series, Phantasm, Bubba Ho-tep, and many others). If you enjoy those movies and are a fan of the genre, then see Slither. If you enjoy comedy and gore intertwined, then see Slither.
Here’s why: Nathan Fillion (of Firefly/Serenity) is the new Bruce Campbell. Sure, he doesn’t have the zany slapstick exploits of Campbell in Evil Dead II, but no one else can deliver one-liners like him. Taken as pure text, the writing would be absolute drivel. Cast Fillion and have him say those words, then you have something fantastic and hilarious. And, as the trend demands, there are countless allusions to previous horror films. I only picked up on a few, but I’m sure that someone more well versed would find the entire movie packed with even more hilarity than I found.