I find it difficult to believe that any phone is better than a blackberry bold or curve at doing email. Not only is the keyboard great for typing but emails arrive quickly, the phone battery lasts a good long time, the phone operation is superb and you should never miss calls with it, even if your phone is engaged with getting emails. Those phones are made for great email and contacts. Just dial your contact's name and it's poetry to use the phone.
K-9 is a great application but, if you're using POP (especially), the phone can have many moments of lag and sluggish behavior. Using the keyboard and the features of K-9 are much better than the iPhone but there is no way I can compare it to how efficient it was to use the BB. The only way K-9 blows away the BB is the ability to more easily look at several email folders quickly instead of using the BB's horrible scroll, scroll, scroll method to click on each email account's icon if you want individual view.
If power email is a necessity, I would never go away from a BB. The Droid provides a far superior browsing experience, more apps, more memory, more overall utility. I do miss some of the great but expensive Blackberry apps and the backup to PC amongst others. Hopefully Froyo and Android 2.2 will bring additional improvements under the hood. Rumor has it that battery life and efficiency are greatly enhanced. Staying positive here...