Yeah they are slow with updates but they make them, while most other developers stop supporting their phones after a few months from their release date (apart from very popular models).
I don't know if there's any other Android 1.6 device that had an official update to 2.3 though. Most developers issue an update or two and that's it.
Being a few months behind doesn't really bother me knowing that I will get that update in the end, just a few months later. Also SE are cutting down on customizations so hopefully that would mean faster upgrades. And their new Xperia line have the newest Android on board, X10 was late at its launch already.
I agree that it would bother me if I had 1,6 when there was 2.2 already. However a friend of mine owns a X10 and he really couldn't be bothered to update to 2,1. I had to convince him and then he wouldn't even see a big difference because the UI is so customized that the only noticeable difference was that some apps didn't work on 1,6 and the update came with hd video recording.
Also don't forget that apart from HTC no other developer would issue more than 1 software update at the beginning of their Android adventure. Samsung i7500 started with 1,5 and ended with 1,6. i5700 started with 1,5 and Ended with 2.1 (barely). Galaxy S was their first model that had bigger support because it was still generating huge profit.
I really respect Sony because they support their phones for a long time and in my opinion they have the best customer service, at least here. I remember when I had my K800i I could count on them even after 3 years of use. I have very huge respect for Sony's mobile phone division but I didn't really like their Androids though (design, UI). However this Xperia Pro looks cool mainly because of a comfortably-looking QWERTY and I decided that my new phone should have a hardware qwerty keyboard after all, and I'm not a fan of Desire Z and Milestones. There are barely any new interesting qwerty models around.