I haven't had that problem, mainly because I haven't bothered with the Steam UI Beta. I've just left everything as it is. If the new UI gets rolled out across the board then I'll use it and just treat it the same way as I did when the Escapist, Youtube and Facebook all changed their UIs. That is, I'll get used to it and forget the old one ever existed. Though if there's an option to keep the current Steam UI then I'll do that, just because I love the current one.
But yeah, I haven't done the UI Beta, so I haven't had any problems. If you reinstall Steam then you should get everything back to normal, and you shouldn't lose any of your games or anything (since they're linked to your account itself, not your installed Steam client, which is the same reason why the Steam Cloud works).
EDIT: Ninja'd by two posts from the same person! And one from the OP! That has to be a record or something, right? Do I get a special badge for that? Please?