I just came back from the theater and so I'll quickly give my thoughts on this, and this is just my personal perception
For the fans that have said that they'd love to see more live vocals instead of all the dancing, this is the closest you'll get to that, this is what you've been asking for.
First off it's important for people to remember that this was only rehearsals, and different stages of rehearsals. Not all the stages are done, not all songs are perfectly rehearsed and there are not slick polished dance moves overrall (no moonwalks or massive spins or anything). What you do get is vocals. Awesome, awesome live vocals, in a way that i've never heard before in a live setting, except maybe the Bad tour. His voice is strong, and sounds just like his records, simply amazing. There are those who argued that songs like Smooth Criminal weren't live, but lipsynced, which was mostly based on clips we've seen. After seeing the whole thing, I can assure you that all songs are live. There are places were some backing is in place, but there are still parts that are live with that. What might confuse you is the fact that his voice simply sounds THAT good
The concepts were simply amazing, from the re-imagined sequences for Smooth Criminal, Thriller and Earth Song, to the awesome city backdrop with dancers climbing it, to the concept for the Earth Song stage opening up, to to luminating Billy Jean jacket, the massively glittering gold pants, the list goes on!. It seemed the clothing was still a work in progress, as Michael used different outfits at times, probalby seeing how it worked an felt.
Which brings me to the man himself. I absolutely love how they showed Michael the way they did, with all his different facets. I went with a group of friends who generally aren't huge fans like myself, so they were so surprised to see his side of him, the simply michael jackson side, the non-tabloid side, the normal side. They couldn't believe how funny he could be, at times he absolutely had the audience in stitches with his deadpan commentary or little moves. I'm so glad they really included this, because it's such a big part of Michael and shows so much of what kind of gentle spirit he is. Even his firm comments and impatiience would be quickly followed by a remark that made sure they all knew he wasn't yelling at them...it was endearing to say the least. We also see the perfectionist, the guy who's hard on himself, the guy who works himself to the bone. He's thin, no doubt about it, and seems at times to be stressed and jittery and tired and slurry, which is not strange for someone buidling this kind of supershow. If there were other reasons, it really doesn't matter. This Is It about the show and how he goes about pulling it together alongside his team of talented workers and artists.
It's simply a great process to see, for the fans as well as the general public. I hope it'll do well. It deserves it. I'm sure I missed some stuff, and others will surely fill in the blanks. I just think it's an absolut winner, as long as you're not expecting some massive finished concert.