I can't live without UMA as I get really bad phone signal coverage across my house. 3G barely penetrates the walls due to a massive University building in between me and the transmitter on the main road. Before UMA I had to stand in my front window and not move a muscle to get a 3G signal. With this Bold and UMA I can use my phone anywhere in the house.
Before the .6xx firmware the UMA was patchy and broke up. Especially with the original .4xx that my phone was delivered with. I use .666 now and will not ever part with it because it works so well. (Sorry - don't know the "bundle" numbers apart from to say that .666 is bundle 2921) I avoid the .7xx out of personal choice as I can see little "niggles" in there that I want to avoid. And .666 works so darn well for me that there is no need to upgrade.
I have my UMA setup on both my home wireless, and with my neighbours four doors down. I am often impressed by the hand offs. Never see "no signal". It is so nippy at swapping I often find that as I pull up outside my house it has already jumped onto my neighbours WiFi signal as I am parking the car as I drive past their door first. Then I walk into my house and it hands over to my home WiFi. All without hiccups.
I have not tried having an actual phone conversation while I am doing this, but certainly get bbm and sms messages appearing before and after the handover.
As to battery pulls - I never do them. Ever. I really cannot recall the last time I did one.