I don't see how it can possibly be the worse phone ever, but everyone is entitled to their own opinion.
I just want to say that I'm currently using the Xperia X10a (Rooted of course) and running on 1.6 Eminence Rom. I could not be more happier with such a great phone:
Beautiful interface,
Runs any programs smooth and fast (Thanks to Jit)
Decent battery life (I don't know why the battery issue has been on the rise these days, but I listen to music on it, text like crazy, and at the end of the day before charge, I still have at least 10-15% battery life left)
Etc. Etc.
I do agree that when I first got the Xperia, I thought it was such a shit phone for the price I paid. It was laggy, touch screen was non responsive most of the times (even with the most recent firmware updates), interface was very "blah". However after rooting it, all those problems went away, and opportunities of utilizing the phone to its max potential is still on the rise! So for new X10 owners, root your phone and play around with it! =)