So over the past 24 hours, I've been considering my opinion on the shift to Focus, from mana.
I offer the following disclaimer; I'm not even doing Heroics any more. My sole two objectives in WoW at this point, are getting a Netherdrake, and the Conqueror PvP title. The 80-85 quests have been sufficiently boring up to this point that it has even been difficult to motivate myself to get to level 85, but I will probably need to if I want sufficient Resilience gear to finish rep farming for Conqueror.
I'm also completely solitary. I made the mistake of transferring back to Jubei'Thos in order to play with my brothers, but they have since moved to Saurfang themselves, ironically; and the community on Jubei seems to be largely non-existent at this point. I tried getting into a guild earlier on, when doing Vashj'ir quests, but got the usual "go go go!" type doing them with me, which I really do not want at all.
My other two activities in WoW will consist of old quests (Nagrand primarily at the moment, for interest's sake) and possibly gold farming for a tundra mammoth, as well. I considered WoTLK's quests to have been the best released so far, and so will probably also finish any of those that I've left also; although due to how much I enjoyed them, most of them were done during WoTLK.
Considering what I use it for, then, most people will probably say that the difference between mana and focus is not all that great. There is a noticeable difference, however. Given that I only used to put Viper on while travelling, (or if I was in really low level areas, where it made no difference) mana meant that I had to drink anywhere between every 10-20 mobs, depending on how much I was trapping.
Now, it really depends on how I play. If I'm not feeling in a hurry, and am happy with simply using Serpent, alternating between Immolation and Snake Trap, and auto shot, I can literally keep going forever. If I use Explosive Shot, however, I will often get down to 50% focus between kills, and so may have to use Steady a few times to get it back up.
Without knowing for certain, I'm idly guessing that will throw a bit of a spanner in the works for those who are doing Heroics or raids, simply because during long fights, there would likely be periods where you'd be stuck doing nothing but Steady (at least for a few shots) with precious little else to do until you caught back up; although given you'd only need 3 Steadies or so, it probably wouldn't be that bad.
The thing I loved about mana in particular during TBC, was the fact that in a 5 man or raid, if I had pots or runes, I could get it back when I really needed it; although I'm guessing that is what Ghostcrawler primarily wanted to prevent us from being able to do, as well.
The really positive side where focus is concerned for me, however, is that with it, traps are completely free; the cooldown itself is my only constraint with using Immolation now. I've removed the Glyph of Immolation Trap which buffed its' damage, as well; at my current level, Immolation has actually become a bit too bursty with the glyph on, and if I'm critting, I can have difficulty keeping threat on the pet.
If I was still doing 5 mans, then, something tells me I'd probably be feeling nostalgic for mana; but for what I do, the occasional per-mob delay is not a big problem, particularly considering that I'm often waiting on Immolation's CD anywayz, if I don't alternate with Snake Trap.