I had issues pairing with the Band after updating to W10 and doing a hard reset. Obviously hard resetting removes the ability to sync, so I just factory reset my band and it paired up fine. No problem with syncing now, works like it did before W10.
It's working like the last build did; everything except band keyboard and cortana work (those just try to connect forever).
I went to Settings > Extras > Device Hub (my phone prompted me to restart to update to the latest Device Hub) and connected my Band that way. It has worked great ever since.
This worked for me. Go to the Bluetooth setting on MS Band and also on your phone. Select Pair on your band and then on your phone.