First, there is always a risk when flushing... Do NAND and EFS before.
As for being stable - AFAIK, from reading and testing, some ICS roms out there are very stable and can be used for daily use. However, you should take in mind that while these ICS roms - as stable as they are - are still new, while GB roms have been outhere for a while.
I think that Slim is very stable. AOKP and CM9 also, yet a little more buggy...
---------- Post added at 05:24 PM ---------- Previous post was at 05:23 PM ----------
Oh, ICS is now on 4.04. Some roms are on 4.03.
Device 1: SGS I-9000
Device 2: SGS I-9300
ROM: P.A 3.15
Kernel: Googy Max 1.31