Does OpenID improve the user experience?
Edit
Not to detract from the other comments, but I got one really good reply below that outlined 3
It's great not having to make too many user accounts all around. All those passwords.... then again, I far prefer a solution like 1Password for the Mac. OpenID is better for sites I'll return to than a separate username, though
I have been finding more and more that if I'm required to pick a username, and my preferred one is already taken, I'll simply leave the site. At least one company has lost a sale this way, and I refuse to join Twitter. On the other hand, sites that use your email address as a username don't have quite the same problem. For me, it's a different problem: Which address did I give them?
The good news about OpenID is that a few major sites have found a way to make it easy for new users to figure out what's going on, by listing the icons of a few sites where they're likely to have accounts. Whether your average user will trust that method is still in question, though.
Maybe it isn't worth the effort on the large scale (yet), but I am very reluctant when it comes to registering on the sites that do not support OpenID: coming up with yet another password, confirming email (which, sometimes, involves waiting for the email), etc. They basically lose me as a user unless I really have a good reason to register there.
But also keep in mind that OpenID is not only about single sign-on, it's the way to maintain your identity, to prove that you are who you claim to be. OpenID sign-on is great, but the ability to perform action on the site on your own behalf (e.g. leave a comment) without registering is even more important.
Personally, I think the value of a well implemented 'lazy registration' concept is far more useful than the OpenId itself.
I already have so many accounts online I don't mind registering on new websites, but forcing me to sign in just to see what the service is about, or to complete an order, is very annoying with or without openid.
What I like most about OpenID is that it doesn't feel like I'm creating an account at all. It's more like I already have an account for the entire Web, and StackOverflow is taking notice of it when I log in. I'm really tired of having to create a new "identity" on every site I run across because they want to have a bigger user count.
I also like that sites that (only) use OpenID tend to make the whole account experience more flexible: no email confirmation required, no enforced-unique usernames, use of Gravatar, etc. The upside is that there is no registration; I just log in like I was already here.
I would agree with you that ease of use for your users is something to heavily consider. Your audience is another thing to consider. As OpenID becomes more accepted this will be less and less of an issue. If you are working on a project where you know the majority of your users will not even know what OpenID is then perhaps you should steer away from it.
Stackoverflow was my first intro into OpenID and I'm a geek.... I created the account after avoiding it for a few days and reading up on it. I finally jumped in but I would venture to say non-geek types would perhaps not. Now, I love the idea and would love to see it everywhere also.
If you can do both your own and OpenID, offer both. I think that would be the best of both worlds. You could point users to the goodness of OpenID but still let them go the other way. If you see a high adoption rate with OpenID you could eventually only offer it.