The negative reviews are stemming mostly from people who can't get over the fact that Segel wasn't involved in this one. Most of the reviews are decent, though, as NJ Dad pointed out it has been "certified fresh" which means that the majority of the reviewers enjoyed it. I will say that if you aren't a fan of the original Muppet Show, it may not appeal to you as much. It's more of a mad-cap, wacky humor type of movie rather than a sentimental heart-warming story that 2011 was. I find the new character Constantine to be so funny. If you go into it just expecting to laugh and realize it's just the Muppets making a goofy movie, I think you will like it. Another criticism I've read is that people couldn't believe none of the gang (aside from Animal) realized that Kermit has been replaced--to me, that's part of the hilarity!
If you have the time, I really do encourage you to see it in the theater. It has had some legs so far but every bit helps.