AFAIK it hasn't been aired where I live, but I've watched the first two seasons on DVD and thought it was good. I heard they got more entangled with the enemy (don't recall what they were called) in the later seasons and that's part of the reason why I haven't ordered more seasons from amazon. I'm not that into big epic stories, I'm more into the stand-alone adventures, like Sliders early seasons and the early seasons of for instance X-files and Xena.
But anyway, I really liked the first two seasons