Grinchmas is fantastic! If you are a fan, it is a definite. We went in early December, so crowds were still low(er) and we booked the earliest Breakfast with the Grinch, and we had nearly 10 minutes with him because there was literally no one else there. Amazing! The show might be blah for some, but as a fan, I thought it was just great. But, the best part for us - every cast member in Seussland played up the part tremendously. Ask them about the Grinch and they completely freak out, so much fun for my kids (and us adults). It started for us as simply asking for directions to the line for the show and turned into a whole different level of interaction
Also, as someone who watched the Macy's day Parade every year growing up, seeing some of the memorabilia around Universal was very cool, and the tree they had (near Jimmy Fallon) was breathtaking. Unfortunately, my kids are a bit older and missing school is a bigger deal now, plus my wife now works in the schools, we are relegated to summer. I would visit every Grinchmas season if I could!