I really like Lena's portrayal of Cersei. Some of the hardcore book fanboys/fangirls on the ASOIAF forum bitch about her, but I think she took a rather one-dimensional, Disney 'evil queen' type of character and made her more realistic.
The main issue is she really isn't sultry enough as Cersei. Not that she's not attractive, but she really doesn't fit the role of someone who is going to have men falling all over her and trying to do her bidding. On top of that she was pretty horrible in season one as Cersei in the acting department as well. She's done a good job since however, she was solid in seasons 2 and 3. But she still doesn't have the look or sex appeal of the character, just the bitchy, haughty part down.
And yeah, Kit is fine as Jon, but the writing for his arc on the show has been subpar.