Agreed that the PLEX looks intriguing, but a big part of the GK sound to me is the power section, so it makes me hesitant. That said it could be a great product. One can only hope that the extended pushback on release dates means that they are working out the issues to make as great a product as possible.
I'm on a bit of a shoestring budget at the moment which is why I was considering the 400RB. It's the same pricepoint as the MB500, which I like fine, but the MB line is just missing that "grunt" that the RB series has. As for the Fusion 550, it seems nice, but two things hole me up. 1) the self-spinning controls seems kinda gimmicky, and to me seems like something else to worry about breaking, 2) for the price, might as well save a bit longer and get a 2001RB as there isn't much difference in price.
Looks like I'll probably be saving up a little longer and holding out for a 700RB or 1001RB.