My fav is resto shaman, although on these fights in can be a bit spammy, but that true for all healers. For me, most fun is shaman, then maybe hpala and rdruid, then disc. Holy priest would be the 2. fun, if it had better tank healing, or I could play it better :D But this is just my personal fun ranking.
I'm used to it. The main advantage is that you can bind every spell without having to do keybinds like CTRL+1, CTRL+SHIFT+1, etc. A CD tracking addon would help a bit though
It is Grid2. I never used Grid, but as far as I know they are similar with Grid2 having more customizability (only thing I miss is mana bars, that would helpful).