I think it could be useful. Personally I love Graphics Gale and despise Photoshop and Painter for overcomplicating things. I mean, seriously, if I make a selection, I want to just click and drag the selection to whereever I darn well please. But noooo. You have to right click, you've got to float, you've got to drag, then you've got to sink the float, then you've got to remerge the layers... AUGH!!! What a complicated waste of time! I just want to click and drag! Why bother with float at all? I could just cut and paste onto a new layer. o_O
Obviously I do not understand professional software. All I need for Graphics Gale to be perfect is for it to have the option to create anti-aliased brushes - or at least, a single brush in there with anti-aliased sides, which would really help with sketching. Can only create monochrome bitmap brushes. Which I guess is more sensible for pixel art, though, so heck.
Would help to have the eraser function on my Wacom pen work there though, instead of it sketching with both sides.
So. Good luck with RMXPaint, hope it comes along well!