It's really unreliable and half assed. The parent commenter is a absolutely right. iMessage is full of bugs they need to really fix them! They have done an amazing thing though.
Sidenote: apple has the ability to mine so many conversations for data. It's more intimate than twitter and I'd imagine more iMessages are sent each day than tweets. If something happens anywhere in the world Apple would theoretically be the first to know. Kinda nuts.
I wasn't aware how buggy Messages application can be until I started using it more. Weird UI glitches, messages coming out of order (?) are the two I notice often.
On both systems, you can't do anything but search for the first instance of a message. It is completely and utterly useless. It also hangs and and crashes on my system if you try.
ED: I tried searching for a word in Messages at five minutes ago, and the results still haven't appeared. 100% CPU usage.
ED2: 10 minutes now, 2.24GB of memory use.
ED3: Results showed, then client crashed at 13 minutes.
Message search is also built into the iOS spotlight screen (left of first home screen).