I have been using Pownce for a few weeks now and love it. I have used both Jaiku and Twitter prior, but find the Pownce system to be my favorite. However, I do like Jaiku's ability to aggregate all of my different feeds in one place. Currently I post on Pownce, use Twitterfeed to post my Pownce feed on Twitter and Jaiku to read my Twitter feed and post on Jaiku.
Here are my 3 feeds:
I have a few Pownce invites, so send me a message with your name and email and I will try to accommodate.