I like Flumphs.
They are actually a very well designed monster. They have unique mechanics. They present a tactical challenge and they have some balance and some level irrelevancy so they that never becomes a complete pushover. Mechanically that's exactly what I want to see in a monster.
The big problem with the Flumph therefore is the fluff, not the crunch. And the problem with the fluff is not that it isn't interesting, because it is. The problem is that it is completely tangental to most DM's setting. It's a monster that feels most comfortable in completely alien science fiction world, not in a traditional fantasy setting. Without an alien world to call its own, it just clashes with the background. Flumphs in a world with unique alien fauna and flora and where everything was as wierd as they were would work fine. Flumphs in a world of dire bears, centaurs, and dragons - not so much.