Many big changes have already taken place in Sid after the Sarge release and many more are yet to come. As for Etch, this storm of big upgrades will actually hit Etch later than Sid (because Etch gets its packages from Sid), but the unstabilizing effect will be considerably smaller in Etch because most of the broken dependencies and incompatibility problems have already been fixed by the time the upgrades reach Etch. Sid will always be a bit hazardous choice for daily use but Debian "testing" (currently called Etch) should be a safe bet for desktop use at all times, especially with apt-listbugs installed.
On the other hand, it will still take a month or two before the big upgrades will be available in Etch, so if you're happy running Sarge at the moment, there are not yet many persuasive reasons for dist-upgrading Sarge to Etch. You can temporarily point the sources.list file to "testing", then run "apt-get update", and then open aptitude and browse the packages that it wants to upgrade. If you don't see any compelling reason to do the upgrade, you can just exit aptitude, point the sources.list back to "sarge", and run "apt-get update" again, and you are safely back in Sarge.
But I'd say there aren't really any stability concerns that should prevent from dist-upgrading a Sarge desktop system to Etch. Server systems are a different matter and there's no point in using anything but Sarge for servers.
