best way to upgrade is with a utility like swaret or slap-get, just take care when you're upgradeing the kernel..
If you ask me, either go with the unoficial slackware current ISOs, or just wait a few days (a week..?) till 10.1 comes out.
As for stability, currently its stable as a rock.. well, tbh, always has been.
Edit: when upgrading from slack 10, you will notice some issues like the change between XFree86, and X11, and their xorg.conf configuration (keyboard -> kbd), etc..., but nothing to worry about. ALL has been discussed and asked before in here.