Just got back from a 5-day trip. We stayed at Animal Kingdom Lodge and and participated in the MagicBand test.
My opinion: The things are pretty great. We did a lot of work before the trip installing the apps and scheduling various Fast Pass Plus times and dining reservations and the bands were sent to us about 2 weeks before the trip. The bands starting working fine from the second we used them to check in to the Magical Express point at Orlando International. The desk staff at Animal Kingdom Lodge seemed very knowledgeable about how to pair them with the tickets we had bought from Undercover Tourist and functioned perfectly as our room keys from the get-go. In fact, the bands seemed to work easier and more reliably than any hotel keycard I've used in the past.
In-park the bands continued to work very well. All our Fast Pass Plus reservations were recognized and the bands were useful for playing the neat interactive pirates game at Adventureland. We had tried the Kim Possible, Phineas and Ferb Cellphone Adventure, and Sorcerors of the Magic: The Gathering Kingdom before, and this one was by far the best of such "games" implemented in the park so far. The one rankle we had was in trying to change our FP+ reservations halfway through the trip when we decided to go to a different park the next day. The app server for My Disney Experience was down for a couple hours on Saturday and Sunday, but eventually it came back online in the evening and caused us no delay.
I read here beforehand that even if you're using FP+ you can still use paper tickets (If you've kept them with you) to get "normal" fast passes to in-effect "double dip" and beat the queues more often. This is still true, at least as of last weekend's test. Hopefully this "feature" will be disabled during the full rollout of FP+, because anyone in the know can abuse the system pretty well with a little planning.
A lot has been said on this board about the "big brother" aspect of the bands and to be honest I think this has been pretty overblown. A few times I had a CM address me by name as I checked in for a park entry or a FP+ return, but I think the real "point" of the Magic Bands is that they make it dangerously easy for a guest to spend more money. You don't even have to pull out your wallet to get your credit card any more if you want to buy a souvenir or snack- just hold up your band, which makes a nice little chime and glows pleasantly (positive reinforcement) and you've given another $13.95 to the Walt Disney Company. This is all very, very convenient, too convenient really, and I can easily see people with little self control spending a lot more money on their vacation because of the way they've built their system.