I'm a HUGE BBM fan. I use it religiously. I've gotten to the point where it's almost annoying to use SMS (BBM fans will know what I'm talking about).
Every cross-platform messenger I've used sucked balls and I hate almost every one of them. I do use Whatsapp to speak to family overseas but that's about it.
I ran across this app just today. It's called LiveProfile. It is VERY similar to BBM. Definitely worth giving a shot. Also, it runs very smooth unlike many other cross-platform messengers. Try it out and let me know what you think!
http://www.liveprofile.com/