I'm enjoying them, too. In fact, I'm loving them. I adore Matt's version of the Doctor. So quirky! And Amy is my favourite companion since the show came back.
Although there are some that don't like one or both, overall it seems most people have taken to them.
I'll miss David, just as I missed Chris and all the other Doctors when they regenerated, but it's par for the course with this show.
And there's nothing really stopping Simm coming back as The Master. The Master always returns, no matter what. He always has!