Installed the official update last night (well, really early this morning) on my Xperia Ray ST18a 1251-4106. No issues so far. Surprisingly, ICS feels smoother than GB for me. I was expecting laggier performance after the blog post on Developer World regarding GB/ICS technical differences but happily, that doesn't seem to be the case (at least with my usage). ICS web browser is a big upgrade over GB browser, plus, now I can use Chrome Beta. I don't have Flash installed. Even on my GB devices, I always uninstall or disable Flash.
Ran Passmark Benchmark. Results for most of the tests were more or less the same. However, 2D and 3D performance took quite a significant dip compared to Gingerbread so for folks who use their phones for gaming, might want to stick to GB for now.