I'm close to half way through season 5 so far, and here's what I think:
Season One was just okay. Not very good, not terrible either. Kind of bland.
Season Two was inconsistent, but when it was good (i.e. The Kazon Arc) it was REALLY good. And when it was bad it was quite painful to watch (Elogium, anyone?)
Season Three was inconsistent like Season Two, but it had a handful of real gems mixed within some pretty bad episodes.
Season Four was the first season, IMO, that was consistently good. Hardly any bad episodes at all. Very enjoyable, and definitely the best season up to that point.
Season Five is too early to judge because I've only seen maybe 10 episodes, but so far I think it's a little bit better than Season Four.
So, in short, I do think the show is improving.