The first two seasons are wonderful. The third season wasn't as strong as the first two. Three main characters from seasons 1 and 2 left the series, and they were sorely missed in season 3. Out of the new cast members added in season 3, I only really cared for two: David Harewood as Tuck, and Toby Stephens as Prince John. Season 3 had a few good episodes; however, I didn't like the direction the last season went, and the writing didn't seem as good. However, in spite of the series going a bit downhill, I still enjoy watching the characters I had grown to love.
The entire cast is excellent. Most BBC series have at least a few actors I'm familiar with from other BBC shows, but other than Richard, I was not familiar with any of the actors. However, I have grown very found of Robin and his merrie band. Since then, I have seen many of the actors in other TV series.
The series starts when Robin (the Earl of Locksley) returns to Nottingham from the Holy Land, where he fought with King Richard. He returns to find that Sir Guy of Gisborne has taken over his lands of Locksley and is over-taxing and terrorizing the people of Locksley in the name of the Sheriff of Nottingham. While fighting to get his land back from Guy, he goes against the Sheriff, and is branded an outlaw.
Jonas Armstrong is a great Robin. He brings a youthfulness, and an energy that is so fun to watch. He is very believable as an action hero, and you can feel his passion to do what is right and fight for the people of Locksley, Nottingham, and in effect, all of England.
After growing to love Richard Armitage as John Thornton in North and South, it was a little difficult to see him as a villain - a murderous villain. However, he is such a great actor that at times he still brings a vulnerability and unexpected likability to Sir Guy. Lucy Griffiths is a very spunky Marian. She was perfectly cast, and I love watching her character evolve through the series. Her character is very complicated, and even though there are times when she is the damsel in distress (Robin needs to be able to save the woman he loves every once in a while), she usually holds her own, fighting for justice and freedom in her own way.
Sam Troughton plays Robin's faithful servant, Much. I loved seeing their relationship. Much had gone to the Holy Land with Robin to fight with King Richard, and he remains a support and loyal friend to Robin through the good and bad times (there are mostly bad times, thanks to the Sheriff and Gisborne). Keith Allen is over-the-top as Vaisey, the Sheriff of Nottingham. His acting sometimes feels like "schmacting" (in live theatre, we also refer to that as "chewing the scenery"), but in this version of Robin Hood, it works.
Gordon Kennedy, is the epitome of Little John. Joe Armstrong plays Allan A Dale, whose journey takes a different direction than his comrades in season 2. This change in direction was great in the story arc of the second season. Harry Lloyd plays Will Scarlet. I'm most familiar with Harry from his great performance as Baines in the two part episodes, "Human Nature" and "Family of Blood" in Doctor Who. The characters of Will and Baines couldn't be more different, and it's great to see Lloyd's range as an actor. Anjali Jay plays Djaq (pronounced Jack), the Saracen who joins Robin's gang. It's great to have a female in the band who can hold her own. Jay is wonderful in the role.
Joanna Froggat (Anna in Downton Abbey) and Lara Pulver (Irene Adler in the BBC's Sherlock) are two characters added in the third season, and I don't care for their characters or their plot lines very much. The actors are still good, but it's hard to watch characters that you don't particularly like.
When you start watching this series, you have to remember that this isn't the Disney version, the Kevin Costner version (thankfully), or the classic Errol Flynn version. This is a Robin Hood for a new generation, and though it isn't completely faithful to the legend you're familiar with (but then, are any of them?), it is a series that has the heart of the legend. And isn't that what you want anyway when you watch the journey of Robin Hood?