Aww, poor
Billy Boyd. I liked Billy as Pippin, mostly because he managed to pull off the adorable naivety that only Pippin (and Sam, a little) seemed to have in the books. I would give him at least a 7/10.
I was extremely disappointed with the way Merry's character was written. Dominic Monaghan managed to pull off the scenes with Eowyn in ROTK well, and his anger in TTT at Treebeard was well-done. I think Dom's problem was a problem of bad writing, because he's an actor of considerable, untapped talent, as far as I'm concerned. I give him a 9/10.
As usual, I give my David Wenham a 10/10.