Are we talking the original BSG or the remake? :o) For the remake I can agree with you on season 1, that season was probably the best Sci-Fi television series ever made. But after that I felt it just got worse for each episode (the boxing filler episode in S2 for example) with the entire final season being horrible and the ending being even worse. Of course this is my opinion and noone has to agree with it. You can probably atribute some of the decline in quality to the writers strike with the last two seasons.
The remake. The writer's strike definitely contributed to the show somewhat going off the rails in the latter seasons, but there were still some absolutely stand-out episodes in there.
See my above comment, I tend to agree with some of the "filler" episodes. I am one of the weirdoes that found the final episode to be perfect. I loved it the first and second time I saw it.