Seasons 1 and 2 were good but 3 and 4 are a mess. Season 4 is particularly bad. They kill off almost all the main characters and randomly replace them with new ones but now there's no development so you don't feel connected to them like you did to the originals. Oh and virtually none of the questions set up from the beginning are answered btw. So I would strongly recommend just the first 1 or 2 seasons. Only reason to watch the second two is for the completionist.
Rufus Sewell must have a ton of back pain from carrying the show. He is easily the best part. Awesome nuanced performance from him.