I had an ST4s for about a year, troued France on it and had a brilliant time. I really wanted a 749 or 999 though and ended up trading it for a 749S (which I also love a lot).
I've seen front disk problems mentioned a few times, and after mine was traded in I was it in the workshop and asked waht was being done. Besides having some carbon cans on it, apparently it needed steering head bearings and the front disks were warped (the mounts were not true). Strange that the same dealer had it for a 2 year service only 2 weeks before I traded it in and never noticed either of those issues. Probbly it's not on the service schedule, but you'd be tempted to thin kit was all OK if just had a major (and expensive) service.
I ended up with ST4 gearing (I think) when I bought new chain/sprockets. I found the front wheel had a tendency to come off the ground if I pulled a way smartish which was .... different. Didn't half go though.
They wold definately benefit from some more open cans and filters. Save a lot of weight too, you;d not beleive how heavy the std exhausts are.
Bottom line is, ST4s is high on my list of bikes I'd own again.
[Edited on 12-9-2005 by HW]