I'm on pownce.
Just started using it, UI looks neater than twitter. Haven't attempted sharing files, so no idea about how it works. Their AIR application looks slick, but think it will take some time to get used to. It somehow doesn't feel intelligent enough. I ended up sending messages, events to people I didn't intend to. Wish there was an API. When they come out with an API, am sure there will be a bunch of nice little apps.
Also twitter being integrated with the existing IM client is a big deal for me. I don't have to remember to launch a new browser page / new client app to see my updates every time. I get them in a gtalk client window. This feature is a big plus for twitter, compared to pownce, for now.
But well, its just a few days since they launched an alpha. So, things will definitely improve. Kevin Rose has a thread open asking for feature requests.
If you are already on pownce, and if you'd care to add me, this is my page: http://pownce.com/arulprasad/
And I got a few invites left. Leave me a comment, and I'll pass you one.