Ouch. The price point comes as no surprise, but the lack of a trial mode is a serious misstep. I've beta tested the app and while it is nice, it needs some polish. I would have regretted spending $10 on it's current state.
Knowing that mango is right around the corner with no other IM options for WP7 users currently, it's probably a smart business decision to force ppl to purchase IM+ instead of making a trial version available. As much as I would like an IM client, i think i'll wait for mango and just use sms for the time being.