I work mainly in Manga Studio 5/Clip Studio Paint (same thing, different name). I detour through Sai for sketching - simply because I used it for so long before I got MS5 that I'm more used to its sketching-tools - but I use MS5 for pretty much all other drawing.
I personally use Manga Studio 4 for panelling and dialogue, because I prefer the way the panelling and dialogue-tools are set up in MS4, but you can do all of that in MS5/Clip Studio if you want. Customising tools, clipping masks, various layer modes - MS5 can really do pretty much anything you ask of it, short of being actual magic.
As for professional endorsement - Peter Bergting, who is currently working as lead artist on Mike Mignola's Baltimore, draws all of his comics in Manga Studio 5, as far as I know.