I downloaded Beluga, a messaging/group-messaging app, to my iPhone mostly out of boredom one day. I've tried Ping, WhatsApp, and their ilk and found them broken and largely pointless. I have unlimited texts anyway, and you couldn't use those applications unless the person you were texting also had them. Trying to get people to download each new app I want to try is like herding chickens.
Beluga is different. First, it has apps for both iPhone and Android, but that isn't all: it has a usable mobile interface for other smartphones, AND it can be used just through text messaging for everyone else. Second, it's useful to me because it works faster than regular texting, especially with photos included, and allows a location to be included with each text. Third, you can carry on a conversation from the app or from a computer: that's right, like IM. Fourth, it's an actually workable group-texting solution. You create groups (cutely termed "pods" in keeping with the whale motif) of people and text amongst yourselves. That's it. Well, not quite it: you can also handily change options, notably notification options, for each pod. So if you are in a pod with 25 people you don't have to hear dings and pings every 5 seconds unless you're just really bored.
It would be interesting to see how much data Beluga uses. I still have an unlimited, grandfathered-in data plan for my iPhone, but I'm guessing some folks don't like these replacements for SMS because they have unlimited texts but not unlimited data. From the speed with which Beluga shoots photos back and forth, I hypothesize they're minimally small, but this issue may be something to consider.
Also, I don't know how Beluga messages with a location and photo appear to the people using pods through regular text messaging (I don't know whether the photos, specifically, show up as MMS messages). If anyone knows, I'd be interested.
Beluga's in beta, and I can't wait to see how it shapes up. It's got a prominent position on my home screen already. If you try it, apparently it's best to sign up through the app rather than the Web site or you get spammed with text messages, which seems like a bad UI decision for a text-replacement system. I never experienced that, but it's something to keep in mind. My friends who did were able to get rid of the duplicate texts by signing in through the app, I think.