I thought Mitchell was a great choice to take over for SG-1. He's got a similar attitude that O'Neill had. I was pretty show that everyone would get back together, but I was sorry to see that Carter wasn't there with them.
I think Vala's character was a good idea. She brings a lot of laughter to the show. Not only that, but her and Daniel make a great looking couple.
I don't know about General Landry though. I don't mind his character that much, but for some reason I don't think he acts enough like a General. I'm comparing him to General Hammond and O'Neill. Landry wasn't a bad choice, but I think they could've done better.