Samsonite,
Roompacks 2 and 6 are awesome and probably the most "hollywood" for lack of a better word, but if you are into eclectic sound, you should definitely check out Mystic spaces pack also...it has some absolutely dreamy long tail reverberation in it. I started with 2 and 6, but ended up buying all the packs eventually anyway I just couldn't help it. The churches they sampled and mystic spaces...have really cool interesting sounds on their own and if you're into eclectic sound I think you will find a lot of reverberation in the tail to work with in those spaces.
Using a hybrid approach, nothing wrong with that, but I also agree with Dietz, you can acheive as lot right in Mir all alone. i prefer to use Miracle or some other algorithmic reverb just for the very longest and latest part of the tail in order to get a bit more modulation and movement and fill in the cracks, but all the early reflections and some of the late reflections too are coming from MirPro, which capture the character of the rooms they are modeling...
I was overwhelmed by MirPro also when I first got it, but after playing around with it for a while it all comes together. Mind you, I haven't tried to create any custom mic tree configurations or anything like that, I don't know enough to do that, and you're right you do kind of need to be a bit of an audio engineer to make use of some of that power. But the good news is you don't really need to, the mic tree presets are already good and Herb made some more presets with stuff like that...and a lot of the VSL tutorial projects have MirPro integrated into them, with mic tree configuration and everything..so you can look at all those and get some good things to work without neccessarily monkeying around with that. How to place instruments in a room is an entirely different matter, especially some of the smaller rooms, I have no idea what I'm doing with zero orch recording engineer experiencel; but the nice thing is you can just move the instrument around on the stage until it sounds good, experiment and find your stage arrangement that works.