Farscape is a fantastic show. Probably behind Firefly in my list of "Shows I'm angry they cancelled".
Farscape is a wonderful show - one of my top 3-4 shows ever. That being said, at least it had 4 full seasons and a full mini-series to tell it's story which I feel it mostly got to do. Yes, a full season 5 would have probably been preferable, but given how hit and miss season 4 was I'm not so sure it would have the best thing for the show.
Firefly was screwed with out of the gate and given some the worst treatment I've seen by the networks to a show in a while. It's really too bad too, Firefly would have been Joss's crown jewel and a brilliant show filled with an amazing, talented cast.