When I first hopped onto the Gurren Lagann train and began watching the series, I had heard nothing about the controversy about an episode that was apparently so badly produced, that the severe backlash against it caused a head exec at Gainax to literally "kick all reason to the curb" long enough to react with comments so offensive that he lost his position. Therefore, I was totally unprepared for severe drop in quality that this episode delivered.
To be fair, the episode didn't do everything wrong. The same awesome voice-acting cast is still there and the soundtrack is as good as ever. But as for the animation, I think my friend put it best when he said, "It wasn't all bad. I liked the opening and the ending. It's just... everything in between..."
To put it simply, after surging through the epic epicness of the first three episodes, the fourth episode almost completely kills the momentum of the show. Think of replacing a pheonix with a pigeon midflight. It just isn't Gurren Lagann. The rough, seemingly low-budget style of the animation completely disconnected me from the show and no matter how hard the voice actors tried, everything that I loved about the characters since episode one was destroyed by the simple bad taste in their visual depictions. Couple that with a semi-generic "monster of the week" storyline, and you have yourself a comparatively terrible episode.
If you're watching this series for the first time, please be aware that this badly animated episode is in fact the only one of its kind, and the series continues from the fifth episode on with the awesome animation style that makes Gurren Lagann what it is. If you're recommending this series to someone, I highly recommend that you give them a heads up about this episode, or even tell them to skip it completely. The last thing you want is for the episode to kill the series for them, and all they'd really be missing is the introduction of a few side characters, something that the sixth episode, half of which is recap, sufficiently explains.
All in all, I'm sure a lot of fans can agree that if you want to fully enjoy the impact of the series your first time around, this episode just might be worth not watching.