I did the M3 update today from the initial 6.1 upgrade (originally had 6.0) and all went well. I had to, as with the initial upgrade to 6.1, restart the upgrade after it hung on the phone download (soft-reset, enable USB in Active Sync, Restart update).
After reinstalling a few things, I tried WiFi (someone else had this problem) and it gave me "page cannot be displayed" error. A check of the connection settings showed it set to "Work" instead of "Internet". Changing it to Internet fixed the problem for me.

I don't see any difference, so far, from the M2 (I guess it was called) update but will keep looking. My phone was stable before but I had to switch to the extended battery in order to go all day without a recharge.

If you want to use the phone as a modem, chk the sticky (WiFi Router #166) with my update on installing Internet Connection Sharing. Disclaimer: You will require a data plan or be charged for usage.