Saw it last night, and despite mixed reviews in the press I thought it was great.
Yes, it's completely implausible, yes it's dumb as hell, but then who ever wanted, or expected, it to be anything other than mindlessly enjoyable popcorn fodder? That's exactly what it is. The cast were all uniformly excellent, and Rampage is so B.A. you'll forget about him not being Mr T inside five minutes.
The pacing is perfect too, wrapping things up in a little over 100 minutes with no half arsed attempts to extend the running time arbitrarily.
Perfect entertainment.