8 - Very Good all around.
Hard to compact a season of 27 episodes down into a 6 episode run-time, but for what it's worth, it pulled it off well. I gave it an 8 as I feel that is realistically what it was worth, I wouldn't have given it anything lower than a 7.
But when compared to the anime series, it is (while there is a decent amount of new content), just simply falls miles short. Miles short and yet I'd still give it an 8.
I'm trying to give a score that is not relative. If this was relative to the anime series, it would have scored in the 5 - 7 region, probably more around 5. I guess that just demonstrates how good I thought the anime was. This was good, the series was better.
If you're looking into getting into Gurren Lagann, the anime series is the way to go.