I've been so damned tired lately, and I do most of my gaming after the wife's gone to bed - so both nights now I've put the game on, explored a little bit, then woken up several hours later with a bad neck and a paused game.
Absolutely loving it though. Still not done much on the mission front. Last night I spent a fun hour or so just fannying about on dirt bikes and dune buggies through the desert and up/down a mountain
As an indication of how much I've accomplished 2 days into possibly the biggest game ever, and I've racked up 10/1000 gamerscore.
(And that was pretty much just 10GS for starting the game).