Tengen Toppa Gurren Lagann: Rasen Shounentan is nothing more than a poor excuse for an alternate setting for TTGL. Poor enough to raise the question if this is just another example of Gainax teasing their fanbase.
Pitiful character development, a disgraceful and broken plot, multiple failed attempts at referencing to the original story, and more loose ends than one can fathom are just few of the many reasons any fan would become enraged at this watered down catastrophe of a story.
As usual, Gainax delivers with stunning artwork and arrangement. However, an eye-candy shell cannot mask the complete disaster this read holds in store.
Confusing plot devices and
broken dialogue makes the reader question what is going on at almost any point in this read. Abrupt character introductions, rushed story development, and pointless relationship changes will leave almost any reader lost in a mess of thoughts as to how the story is progressing.
The manga spans over the course of presumably nothing more than a day, yet relationships that are built between characters are destroyed or changed almost immediately. These changes in character bonds are completely over-dramatized and completely unrealistic. How can the reader feel anything for a character losing a relationship that has lasted presumably nothing more than a few hours?
Confusing age differences between normally closely aged characters, teams made up of less that 2 people, and an ending that leaves the reader unsatisfied and confused leaves a shameful mark on a brilliant series.
If you are looking for an interesting spin on Tengen Toppa Gurren Lagann's storyline, I recommend you look at fanfiction.net. I guarantee that there are dozens of more satisfying takes there.