I have a Google alert set up for the word Pownce. So everyday, I get a sizeable list of articles and blog posts that mention Kevin Rose's new project... Mostly, I skim through a bunch of people offering Pownce invites through their blogs (alright, I have some invites too; leave a comment if you're in need).
Pownce, like Twitter, is a microblogging service with some snazzy features--like file sharing (limited to a certain amount of space, with a larger amount for the upsell). To that end, Pownce seems to have more options for functionality (file sharing, link posting, etc.), but I'm still tweeting for Twitter.
Twitter is simple microblogging service. You have 140 characters to make your point; short and to the point; terse. It doesn't have the bells and whistles that Pownce has, but if I found a need for 'em or I would have crossed over. Moreover, all my friends are on Twitter. And there isn't much motivation to move to another similar service and rebuild net communities that are already in existence elsewhere.
FYI- I'm sandyk at Pownce and skalik at Twitter. Be advised I'm on Twitter all the time (even now) and I only check Pownce once a week, if not less.
Jargon Jar: Microblogging
I think of microblogging more like instant messaging to the masses than to blogging. Instead of sending a direct message to one person, you post to a microblogging site (Twitter, Pownce, et al) and every one of your friends / followers sees it. Most sites have direct message options, it's the exception rather than the rule.
Generally speaking, microblogging is a tool for quick, often up-to-the minute, updates. Posts range from what the person had for breakfast to shout outs to new friends or followers and syndicating blog posts (i.e. "check out my post on this at www.tinyurl.com/339gj4").
Shameless self promotion is encouraged... But I suppose when you work in PR, you ought to expect promotion of all kinds.