First of all, the upgrade to 2.5.1 is really good to me. I think it all has improved a lot!
There's however a small problem in the file "wp-includes/class-phpmailer.php"
At line 156 it sets the variable $Host at "localhost"
Now this will be good for 99% of the users, but not for everyone. The host is already defined in the config.php, and so it would be better to get that variable from there.
For now I've edited the file and changed "localhost" into what I need, but this means that with every upgrade, I'll have to edit that cause otherwise I won't be able to send out emails.
It would be really nice if this could be fixed in the new WordPress Release :)