I also have a 550. I bought it a few years ago to learn to play piano and kinda break into music production/keyboards. If you already know how to play piano well, the synth action keys will probably dissapoint you. I really dont like to play serious peices on it. However, as a midi controller, it works just as I'd want one too, and occasionally I'll sample some of it's pads or instruments, because some are quite good.
So, basically, as a newbie board it's pretty good, but stepping up to something more... proficient is more reccomended.