Some of us have had our S950s for nearly 5 months and have been gigging with them on a daily basis. Most of what Donny has posted is general knowledge. For those here who own the S950 I have tweaked nearly 500 styles including the Tyros 4 styles which will play very well on the S950. I have also done some registrations. All this can be found on the PSRtutorial website.
Yes, just about all of the styles from older keyboards play well without much if any adjustments. But, hasn't that been the case with all of Yamaha's keyboards.
At home I use a passive USB hub and keep two 16 gig thumb drives in it. I use one as a backup and keep it updated anytime I make any changes to anything. I always take both with me to gigs.
As Donny mentioned, the USB slot is not very well positioned and could be bumped and cause a problem if you are not careful. What strikes me as strange is why Yamaha did not continue using the same memory card setup as found on the PSR3000. Using an SD card which could be left in the keyboard would have solved the USB problem.
I disagree with Donny about the sax sounds - I think they suck as compared to all the other Yamaha keyboards I have owned. They are not full enough and often sound very weak. It is not a volume problem but seems to be a sampling issue.
All in all I think the S950 is a very nice keyboard - light weight - small foot print - and is about as close to the T4 as your going to get. I have not yet had any buyer's remorse.
Deane