I rewatched this last night (bad idea, I got no sleep as a result) and I thought I'd share a few impressions.
Giamatti and Linney render this almost unwatchable. Their put on and bizarre accents reek of actorly affectation, and sound nothing like what people of their station in their time would have sounded like. I am not a fan of either, as actors, though of the two I have a greater tolerance for Giamatti. He can bring some interesting stuff to some roles, but this is one he was far from suited for. He is someone who really is not convincing as anything other than a 20th or 21st century person.
I wish HBO had gone with a more ambitious project, like a more generalized Birth of America mini-series. Adams is one of my least favorite founding fathers, despite his Massachusetts heritage. I'm more of a Jefferson girl, what can I say*
Anyway, a few quick notes:
MVP: Stephen Dillane, who turns in fascinating work as Jefferson

Runner up: Sarah Polley as A daughter of John Adams (despite her unconventional heritage for such a role**, she equips herself well)

Colonial Williamsburg is a good asset, but there are only so many ways they can re-dress the same buildings before "Philadelphia" becomes indistinguishable from "Boston". Now, HBO previously did great work with redecorating the same sets on BAND OF BROTHERS and ROME, changing locations from France to Germany, and Rome to Egypt while remaining utterly convincing. I have to imagine then that the similar looking sets on this one are due to restrictions Williamsburg placed on how far HBO could go in their alterations. So maybe a bigger investment (along with a non Adams centric focus) where they built sets might have payed off in more varied action (you could have exploding buildings, for instance!)
Oh, and watching it all in one night, the mini felt like it had a right wing tilt to it. Adams is a favorite of right wingers
PS The one scene where I like Giamatti's work is when he meets King George.
PPS: Giamatti's make up was terrible. It never looked like he was getting older, just like he was coming down with AIDS or a meth addiction. He needed some wrinkle prosthesis, not just liver spots and bad teeth
PPPS: David Morse as Washington was pretty good. It was kind of creepy how similiar they looked at times
* My dad despises Jefferson, and calls him duplicitous
** She is from Canada