I have been using the beta for about 4 days now, but I'm a new user of ONE so I might not be the best test candidate. Anyway, so far it's going well. The only issue I had was when my PC went to sleep one time with TOS and ONE open, then when I went back in, the pricing in ONE was all funky and did not line up with TOS. To fix, I closed ONE completely and reopened it and everything was fine.
I want to add that when upgrading from the regular version(just install the beta without uninstalling the reg version), I didn't need to do anything special to retain all my test trades and accounts. Those stayed in place so I was happy about that.