My experience is limited... I had a 650sx for about 6 months but only rode it three times. I did have a 550 for some time but that was years and years ago when I was a lot younger.
The problems:
I can get up, stand up, but as soon as I turn I fall off... Well... Most of the time...
There are times when doing figure of eights it sort of comes together but then I will come out of a turn, hit my wake, the nose will go up then a little further, then I come off as it tends not to have the power to pull he out of it. Since matching and smoothing out the shoulder behind the prop, it has been much better and I am running a Skat 19/24 that has been tweaked a little which also helps. I have a very strong engine also but I am stuck with the basics on this ski. I must say one thing though... I can ride an sj now after spending some time on the FX...