Wow...a 2 movie weekend, Charlie and Choc Factort & FF!!
For 2/3 of the film, I had a good time with the film, but having to balance the levity by the FF with the the Dr. Doom segments made the last third of the film very uneven.
The interplay between the FF members was pretty good, much of it was true to the spirit of the characters in the comics. Johnny Storm provided most of the yuks, and usually at Ben Grimm's expense. I can't say there was great on-screen chemistry between Reed Richard and Sue Storm, but it wasn't horrible.
After the "origin" segment is over, the film sort of putters along, but it never gains a lot of traction, so the final act is average, not terrible, but not great either.
I give it 2.5 stars, or a grade of C+.