It's nowhere near as bad as you've been told, that said, it's not amazingly perfect either, this coming from one of the biggest fans of the show out there.
Goseiger seems to have suffered a bit from behind the scenes issues that threw the show into limbo when it came to advancing the plot, this is most evident in the second arc that centers on the Yuumajuu. I wouldn't say it never recovers from the nosedive, a new producer steps in around the final 17 or so episodes and there is a noticeable improvement in the series. That said, it seems like a last really good quarter preceded by a sprinkling of really awesome episodes throughout the rest of the series wasn't enough for most people to get into.
I've found that if you see yourself enjoying the characters, you can probably truck on through the more underwhelming bits of the series. The early episodes in the second arc are pretty good, but it just hits this brick wall after that I think most people just can't get through. I'll go as far as saying that despite what went on behind the scenes, it's a pretty great series outside of the Yuumajuu arc, and even then, the early episodes are interesting. I think one needs to go into Goseiger with the mindset that this is a more character focused series than plot heavy one.