My first reaction when I heard that they were bringing FLCL back was one of excitement, which quickly gave way to fear. It turned out to be exactly what I was worried about.
It's really difficult not to compare it to the original. It is a sequel nonetheless. In reality, there is no comparison. The original was incredibly unique, emotional, character-driven, well thought out, and had an amazing soundtrack. Progressive is none of those. It is very generic, bland, and lacking any kind of interesting characters or development. While The Pillows are back to provide most of the soundtrack, it is mostly reused from the original and their new tracks lack emotion, even just as background music.
The animation quality is pretty low and varies drastically from episode to episode. The voice acting is seriously lacking, but it's hard to tell if it's the acting or the crappy script. The story is confusing and boring at best. Much of this stems from the poor character development which is virtually nonexistent. I had absolutely zero emotional attachment to any of them and still do not understand what their motivations were at all.
It is clear that FLCL was meant to start and stop at a certain place for a reason. It was planned from the start to be a 6 episode mini-series. Progressive is only non-canonical filler and it feels just like that. It is virtually impossible to bring back this kind of show almost 20 years later and try to capture that same lightning in a bottle. For many people, the original series is up there with some of the best anime and they truly have fond memories of watching it for the first time. Progressive only cheapens that.
Honestly, I think if the next installment, Alternative, wasn't already made, it would have been cancelled after how seeing how bad Progressive turned out. Overall it is not the worst thing that I've ever seen, but it is most definitely on my list of most disappointing sequels and things that should never have been made. Completely unmemorable and forgettable.