I actually watched it for the first time recently and quite enjoyed it. Lots of interesting characters and story. Like already mentioned the CGI is really bad. I wish they stuck with models rather than early 3D effects. Season 5 and the end of 4 feel a bit underwhelming after the main threat is resolved sometime in the middle of the penultimate season.
As an FYI, they didn't expect to get renewed for S5 so the end of S4 was originally supposed to be the series finale. Then they had to come up with a new one