We saw it today. I definitely don't think it was bad. As a stand alone film, I thought it was cute and enjoyable. The music was underwhelming. I can't imagine any of the songs catching on as Disney standards. This was my first time seeing Lin Manuel Miranda do anything and I enjoyed his performance. However I didn't care for Emily Blunt at all. If she was supposed to be playing the same character as Julie Andrews, it sure didn't show.
As for how it was as a "sequel", I don't think it did well in that regard. In fact, I really wouldn't call it a sequel (hence the quotation marks). It was really just a retelling of the original story. I saw one comment on Facebook that matched it up against the original scene for scene and that is very true. I think I would have liked it a lot more had they not gone that route and had come up with a brand new story instead. The whole time, I'm thinking "Oh, this is the new rooftop dance number and this is the new bedtime song and this is the new tea party on the ceiling scene". Both my daughter and I found that blatantly obvious.
So overall, I think it is worth seeing and I enjoyed it more than I thought I might but I don't think it will have any staying power.