I have just tried it and as a preview version I must say it's promising, taking into account that it's based in twhirl, my favorite twitter client. For example, so far, you just can publish text-only status updates.
Here's my short feature wishlist for Seesmic for facebook next releases:
- Followint twhirl naming scheme, I would rename Seesmic for facebook as facerl;
- Replace Seesmic icon with something else more facebookish, like twhirl's icon but in blue hues;
- Minimize to system tray;
- Ability to publish pictures with optional captions, links and notes;
- Ability to like and/or comment on a friend status update;
- Ability to write on a friend's wall;
- Show online friends (counter and list);
- Ability to initiate a chat with a friend;
- Ability to load our custom friend lists and use them as filters;
On the other hand, if you prefer a multi protocol desktop client with this level of basic facebook functionality, I'd suggest trying Digsby, though that's only for Windows for the moment.