Being a long time BBM user, I was looking for something to replace it. I tried LP, Kik, etc. I think that Kik beats LP hands down on reliability.
Kik works great 99% of the time, while LP would work about 70% of the time. I got iPhone users, and BB users who didn't get my messages, had disappearing messages, had other issues (like showing that I'm not connected when I actually was).
My biggest issues with Kik was not being able to message pictures from the gallery or group chat. They fixed both of those issues in the update at the end of April. I'm a believer.
Now all I need is for it to work on Blackberries.