Agreed. Although Persona's technical basis and privacy protections are second to none, the UX is nothing to write home about. It still feels too much like OpenID, and we know what happened to OpenID. Facebook and Twitter can get away with cross-site redirects because they're well known and people trust them. Persona doesn't have that benefit, so it can't get away with the same cumbersome UX. It needs to do better, much better. The market is unfair. Deal with it.
If you're in the business of implementing an alternative login system, you should also seriously think about what kind of UX you're competing against. Your ultimate competitor isn't Facebook or Twitter. It's the good old email-and-password login system that everyone is used to. You enter your email address, select a password, and you're in, without ever leaving the signup page! It's even easier if you use a password manager like LastPass. That's what you're competing against, and if your UX has any more steps or redirects than that, you're probably doomed.