The only issue I had with Christensen's performance was his voice
--it didn't have enough gravitas for me (Matt Lanter, who plays Anakin in current The Clone Wars
cartoon, does, IMO).
Otherwise, I think Christensen's facial expressions
were spot on and did cover quite a range of emotions very well. Two key moments that stand out for me--when Anakin confessed to Padme his frustrations following Shimi's death and revealed how he slaughtered the tribe of Sand People in Episode II, and a blink-and-you-miss it moment in Episode III on Coruscant when Anakin and Obi-Wan part ways and we see Anakin's expression turn from light to dark (we see it again when he is reunited with Obi-Wan on Mustafar).