Especially since the over-the-top Lensman arms race, humongous mecha, fanservice (aka the "Gainax bounce"), and a fairly solid usage of relativistic time dilation makes an odd mix that really shouldn't work as well as it does.
Fortunately the series is sustained by solid character development, the fanservice is muted once things get serious, and the Rip Van Winkle effect on the characters of the time dilation is fully explored.
Gunbuster is an old series (20+ years) that shows its age the most in the earlier episodes where the twenty minutes into the future syndrome results in the display of some truly dated technology (tape drives and tickertape printouts when you have SSTO airliners...). Conversely there is also some eerily precognitive technology in there as well - the photo messages that show up a couple of times are very close to ipods.
In later episodes the technology looks a lot less dated and this backs up the relativistic effects of the plotline. Although the uplifted dolphin in episode 6 never really gets explained and is just there in the background.
The proof of the character development comes in the last episode which is still an effective tear-jerker even when you know exactly what is coming.
Finally, at only 6 episodes, Gunbuster is also short and the action is well paced (see my earlier post on less is more, and we'll quietly ignore my fondness for shows where nothing happens very beautifully).
Still one of my favourites even if I do still wish that Noriko would just grow up half an episode faster. :)