I was on Dream for the 10/23 sailing. We used it.
With kids still aged to go to club/lab we will have to carry the wave phone, too.
DS has a Samsung Galaxy S2 without sim card. It can get wifi, though. As we were sailing away we were madly downloading the app on his device and on DH's whatever-he-has. I already had it on my iphone4. So I just had to relaunch with the ship's wifi.
DS took it to the kid's club. When he was ready to go, he used the app to let me know. That. Was. Awesome.
When using the wave phones it can take AGES to get the message. At least for us. By the time we would get there sometimes he had gotten unbored if that's a word. The app made it much easier for him to let me know he was done. (and he really didn't enjoy the Lab this time, having just had a total blast on Vision of the Seas right before Dream, so he was contacting us pretty quickly!)
We carried the wave phone once, can't remember why. Didn't use it.
So if the kids have any devices that can use wifi, and if they are old and mature enough to carry them with them, it's very useful.
Does this mean texting may/will occur across/between Android and Apple products aboard, using said App?
Including Ipods?
Our family shares platforms. Kids are Apple, parents are Android.
I have apple while DS and DH have android. It worked seamlessly between us.
Also, it ONLY vibrates. No noises from it.
The ONE time that the system was difficult is when my guys were at the movie. They wanted me to know what they were doing and because a phone's screen is a million times brighter than the wave phone, they were chastised for it. (why DH couldn't just step out I'm not sure, and why the CMs cared since he was at the back not near people I don't know, but that was a downfall of it)
It's hard, once you're used to a certain level of communication, to go to a lower level of communication, so that was a bit of a growing pain so to speak.
I got the email the other day and it mentioned a PIN that an under-13 child would have to put in order to use. I can't find any information on where that PIN comes from... anyone know?

We're cruising in a little over three weeks and plan to use the app to communicate with DD (11).
When you're setting up the accounts, it sends a pin to the adult's phone to use on a minor's account. I assume it wouldn't do that if you don't have a minor in your room. We did, so it did.

VERY easy.
This actually does not please me. I love being disconnected onboard. Having a cell phone is too much temptation to just sit and use it all the time instead of enjoying other people.
The app wasn't THAT tempting.
But having the entire cruise's navigators at my fingertips? THAT was useful and took up some of my time. You could even mark things as favorites, and it would pop up those things when it was time for them. Very very helpful.
Other than that, nothing else called me; I didn't have an internet package so those were the only things nudging me.
Bummer. We'll try it week after next, too. Hoping it works better than 2 days later!!
FWIW, the Wave phones were really unreliable for us. The app worked a lot better, though I also made sure to go TO the app every so often to make sure I hadn't missed a buzz from it (it vibrates and I don't always hear that).