I dont think it's all that crazy if you look at how the average rating is moving.
Looks like most critics are rating it between a 5 or 6 out of ten, with more rating it towards 6 as more are seeing it, with the audience rating being a pretty solid 8. Early audience viewers will probably skew high due to the fact that they probably like Marvel more than the average movie goer (I know that goes for me when I saw it last night to a pretty packed house).
I'd say a 5 to 8 range is probably where most people would fall. I would be surprised if most people loved or out right hated the movie. it is slow, its a new arm of the MCU so there is not a lot of connective tissue, some characters are better than others, the cosmic stuff is pretty cool, the cast chemistry is pretty good, movie is a solid "fine". I imagine most people who would rate it lower than 5 would do so due to it being over 2.5 hours long, and still being just fine.
I would rate this and venom 2 pretty similar in quality, for example, but I would rate Venom 2 higher, since it was so short, I didnt care as much that it was just "fine"