Your mileage WILL vary. If that's the most negative review there is, it's still an enjoyable movie. Prawncracker seems to have exploded with joy, and everything else we've heard is strongly positive.
I'm not expecting to see anything better than The Muppet Movie or Muppets Take Manhattan or anything like that... what I am expecting is something much better than MFS... and even then, while that was a completely flawed film, I found it pretty enjoyable. I'm not going into this thinking the Muppets are back (as I don't think they've gone anywhere), but I'm going into thinking I'll pretty much love any Muppet Movie on the big screen. heck, I even dug Elmo in Grouchland (though still had problems with it). Honestly, I may even just eclipse my standards and dare to expect it to be the best post-Henson Muppet project (Best post Henson project outside the Muppets? Dinosaurs! Go on... tell me that wasn't an epic show that said things that needed to be said. Tell me it wasn't amazing, and I'll ignore you).