The plot to this movie is really as simple as it gets, but the fact that it was a mix of a parody and tribute to classic alien invasion films made it work. It may have its slow bits at times, but it wasn't anything that brings the movie to a complete stop. Despite its pacing issues, overall it was an entertaining story for what it was and was fun to see the story unfold.
The acting in the film was great all around in my opinion. I always love Nathan Fillion in pretty much any role, and he was outstanding here as Bill Pardy. His style of acting worked well here in this setting and he delivered a believable performance. Elizabeth Banks was good in her role as Starla Grant, wife of Grant Grant, the infected host of the alien at the center of the story. Grant Grant was played by Michael Rooker who was tremendous in this film and acted out being infected and his transformation over time very well.
Is slither a scary horror film? No, in fact I wouldn't even say its a serious one. What it is, in my opinion at least, is a fun tribute to b movie horror films of the 50's and 60's. The pacing may be off in spots, but considering this was James Gunn's first film as director I would say it turned out well. It has scares, laughs and dramatic moments, and it is entertaining the whole way through.
Verdict: Fun horror film to kill a few hours with