Warning: There are LOTS of spoilers in this review for the movie and the anime series (because I will be comparing the two) so if you don't want to run into any spoilers, do not read any further (except the last paragraph where I summarize all my points in the review).

Plot/Storyline

The first thing shown is a prequel to the series. We see what happened before Lord Genome became the Spiral King. If you have not seen the series yet, this segment may be a big confusing because it goes into it really fast. Fans of the anime will appreciate the additional scenes if they haven't seen Gainax's Parallel Works ("All You Bastards, Get Fired Up). If you have seen that music video, it's similar to it except a more abbreviated version.

The first few scenes of the movie paces itself well. There weren't that many new additional animated scenes tthat go with the existing ones from the anime but they cut out enough to make it understandable for a first time viewer of the franchise. Episodes 1-3 were excellently spliced together.

The scenes that corresponded to episodes 4-6 of the show was horrendously done in the movie and I believe it is the worst part of all. Basically, all they did was put "Row Row Fight the Power" on and picked random scenes from the episodes and smacked that right into the movie. Most of the time, it was dialogue-less so unless you knew the series beforehand, you would get lost. Characters like Rossiu, Gimmy, Darry, and the Black Siblings had no introduction and there were no additional scenes animated (that I was aware of anyway). It felt like a really bad recap episode that companies use to fill in more episode slots (except this is within the movie and there is nothing to really recap). Explanations for these characters to be meeting with Gurren-dan were not given and basically they hope you'll just want to skip the "filler" episodes by zooming past it in the movie.

When versing Viral (and eventually Thymilph), there were some excellent additional animations and they did not cut down too much where people would not understand. There wasn't that much backstory why they were fighting the humans but it was enough to have the basic plot of Gurren Lagann down. The only key thing I found within this fight was when Kamina forgets who Kittan is. It does happen in the anime but it seems more appropriate in this movie because the viewer barely gets to know Kittan from the rushed job of the "filler" insert of the movie so it was more hilarious then intended. Kamina's downfall seemed just as tragic as it did in the series.

One of the scenes I really appreciated is when Nia talks to Simon while he is sculpting Kamina statues. As Nia provides her version of consoling Simon and how he does not need Kamina, Yoko gets uppity about it. The little bits of conversation additions were transitioned nicely and they summed up parts of Nia's history well without rushing anything.

The most epic part of the whole movie is roughly the last quarter of the movie. The battles between the remaining generals were combined into one huge battle for the movie. Almost all of the scenes here were new animation. It gave a refreshing new scene to the series without deviating far from the series. Yes, each general had their own battles in the series but the movie battle was nicely animated. This is the most memorable scene because it is different from the show and it makes it seem more impossible to win against the generals. Simon seems mot mature because he tries to save Nia in the most heroic way without using Lagann.

Another scene I appreciate within this large battle is when Yoko fights Adiane. In the show, lots of people criticized that Yoko was basically useless after a while because her gun did not do anything and she's just the "doom bitch." This small additional scene gives her more screentime to show her badassness and that she can hold her own in melee combat. And of course, there's a lovely fan service shot (kinda) where Adiane rips Yoko's bikini top off so that should be appreciated by all the guys who watch the movie.

Animation

As I have stated already, most of the movie had spliced anime scenes so that can be a bummer. It doesn't really slow down the movie because they pick good ones to make the whole movie flow (other than the "filler" one).
The new animated scenes are amazing. Gainax kicked it up a bit because of fan expectations. These scenes were action packed just like the rest of the series. The backgrounds were gorgeous and the lines just moved effortlessly.
The ending credits had a new animation as well with Simon walking with Boota along. It sort of mimics the ending theme "Underground" by High Voltage except that Simon changes his pace and stature as he walks toward the spinning spiral.
The scene after the credits actually prepares for the sequel movie nicely. There's enough of a cliffhanger for those who won't know the series to continue to the next movie and fans will appreciate how it mimics the "next episode" sequence after an episode's ending credits.
Another good point to mention is who doesn't want to see Yoko practically naked twice in the movie and a panty shot of Nia? What better way to rewatch the series by seeing these majestic shots alone.

Music/Sound Effects/Voice Acting

There really isn't much to say about sounds within the movie because a lot of it is recycled from the anime. Each piece of audio was picked well to convey the emotions within each scene. The memorable quotes from the anime are in the movie so the viewer can hear the awe-inspiring, motivating speeches. Music was placed in the correct places to help show feelings.
As for the new stuff, the ending credits has a new song and it was decent. It did not stand out compared to other ending songs Gurren Lagann has but the song fits well within the series. The voice work for the few additional scenes did not differentiate compared to the previously recorded things so it did not stick out like a sore thumb.

Spoiler free summary of my review:

Basically, the first half of the movie was alright but it really shines when it gets toward the last battle in the last quarter of the movie. The small animation additions here and there help quicken the storyline without losing too much of the core plot and the entirely new scenes really make up for watching all the reused animations from the show. I recommend that if you haven't watched the anime series to watch it first because there are parts that are confusing but for the seasoned fan, I highly recommend it (but you should really just skip the first half of the movie unless you want to watch recapped scenes with minimal changes).