I've used it twice since i got my bike, wanted to see how much power it took off the bike. I have never had any problems in the rain, and never took it out of A mode. the power comes on so smooth you don't really need anything else in the rain. the power is just already so easily controlled in A mode.
thats whats so great about the busa, its awesome at traveling 180mph, but at the same time its perfectly happy cruising around with your Harley buddy's as well. It doesn't matter how fast you drive it (or how slow), or how you accelerate, the busa does a great job what ever your riding style is.