I'd never seen the show before, but decided to marathon the whole thing beginning around Thanksgiving because Netflix has all 5 seasons available for streaming in HD (via my PS3).
Loved the pilot and really enjoyed season 1, liked season 2, was lukewarm about season 3, thought season 4 was rather weak, and found that the 5th season brought the show back on course.
Overall, pretty cool show. I'm looking forward to season 6.