I've used Native Instruments Massive primarily on a friends system who I work with. He's got a decked out Logic set up and we work on his system when we're working in the box and mine when we're doing actual acoustic recording. I don't know all the specs that he's running, but it's an Apple G5 computer that's running both Logic and Pro Tools, although we only use Massive in Logic. It's his go to virtual synth and we use it on pretty much everything we do together. You definitely need to have a good system to run this!
If you're looking for the ultimate virtual synthesizer, Native Instruments' Massive is for you. I'd highly recommend checking out Komplete 7 as a whole as it's a nice savings when you break it down per price per piece of software included. Massive is probably my favorite of the bunch, but each does it's own thing and everything included is quite useful. Massive on it's own has some of the best digital synth sounds I've heard, as everything from the leads to the bass synth sounds are great. You can tweak to your heart's desire and make your own sounds as well. It has all sorts of effects, filters, and everything you'd want when getting your synth sounds. Massive really is one of the best when it comes to virtual synthesizers.