Some folks seem to like the Wusikstation - which the original version is like $10 now I think...
I really like SonikSynth2 from IK Multimedia/Sonic Reality (mentioned above as a thingy I think [;]) ). It is desgned to be a workstation synth, so there's a lot of everything in here, but its emphasis is on synth sounds. It uses the "sampletank" engine, which allows a pretty good degree of manipulation and effecting as well as multi-output and multitimbral capabilities within a single running instance - like the Edirol VSC, but with a quality level that is WAY beyond it. There are gigs of total samples from mainstream stuff to really esoteric and rare.
I know there are some who don't think the library is all that, but I quite like it and I imagine it to be in the same class as Dimernsion and I think sounds quite good - and it's a wellbehaved and easy to use app.
Also, if you get the whole sampletank product, you can load the sonik synth stuff into it as well as other compatible sound sets, and right now the sample tank program has a few more options than just the sonik synth2 app - thought ss2 is quite capable on its own.
You can get it at esoundz.com (or a bunch of other places I suppose) - they are good with customer service and whatnot
I've been looking at getting Dimension Pro, since I've received some sort of promotion from Cakewalk...but I have more important places to put my money right now, unfortunately.
Hope that's helpful.