Not sure why it works bettter than native app. I even get it almost instantly by settiing it up to check every minute and it shows you on the notification bar when its checking.
i can tell you now that yahoo doesn't work on this still. so not sure if it's yahoo side or droid side, but even this app won't connect to yahoo via imap. (yes, using those settings in the tutorial)
at least at this moment it doesn't cause i tried, though yahoo seems to work in spurts so it may when you try it.
K-9 is a really good email app. I love it. If they had a way to collapse the To/CC fields so it doesn't show ALL the email addresses in there, it would be perfect. Other than that, it works very well with my Google Apps email, and it supports flags (star on Google Apps Mail/Gmail) displaying remote images, and the interface is very very well designed.