I actually enjoyed the film and laughed my butt off at parts of it, but yeah the plot is pretty weak compared to the original. The 4 leads make the film enjoyable and Chris Hemsworth is hilarious.
I totally get the rotten audience score on RT. The film just leans way too much on the original to prop it up when I think some of the brightest moments are from where it differs from the original.
Clearly the film makers intended this to be more of a tribute film than a remake, with cameo's from most of the surviving cast, sans Rick Moranis who declined to make an appearance.
One thing that did bother me was the Batman Begins type explanation of how they get everything, like the GB logo, their uniforms, etc. Totally unessesary.
There's a dance number that appears in the credits that you expect to happen in the film. Totally wasted moment there, because it was hilarious.
Overall I'd give it a B-/C+ and hope it does well enough to spur the sequel teased in the after credit scene.