Overall: My Best Friend's Exorcism is lightweight and fun, with a few great moments, but it felt a little bland during some sections. It was worth watching, but it's pretty average.
I wanted to like this a lot more than I do because they got some stuff *so* right, but overall it felt pretty lackluster for most of the film. I think it's a solid 5.5/10 and if they had made some of it a little less generic it could have easily hit a 7 with the things they did right.
The good: They have a couple of awesome scenes/effects that got me. They've got some good jokes. They have a fun tertiary character, but unfortunately don't use him enough. The production quality is great, all the actors seemed ok, and the overall tone is quite fun.
The not-so good: They spend a lot of time slowly panning through bland scenes and not adding to the film. They don't have quite enough characterization outside of "these girls are quirky friends", "these girls are shallow bullies", and "these jocks are soooo mean". They didn't lean in enough with the side characters and some of the more interesting bits, and spent a lot of time on cookie-cutter stuff that wasn't memorable.