I have the Mastering Bundle (I find it indispensable for, well, mastering) but I've largely eschewed the MSpectralDynamics plugin because I didn't know what it was for.
A couple of weeks ago I dropped in on the Bass guitar track just for kicks (I had nothing to lose, the bass track wasn't sitting right and I was planning to tweak it). What BitFlipper describes in his article is exactly what I experienced: The Bass suddenly sounded great in the mix. I was embarrassed that it was so much better and I really had no idea why.
I know this sounds like one of those stupid fake endorsements... but
"I used it and I couldn't believe what it did to my Bass track" - some random audio tech Definitely worth a look. Only you can say if it is worth the $$.