I had seven e-mail accounts on my BlackBerry when I was using it. It was nice to have the unified message box and to be able to choose which account to send from once creating the e-mail.
Now, I'm still using the default GMail app (and have one personal POP3 account merged into the GMail account - seems to work better for that one account).
For the other accounts, I have switched to K-9 Mail. It gives me a unified inbox for all accounts, plus I can go into each account separately if I so choose. I can't create a message, then select an account to send from (it will select the account I set as default instead), but I can go into any of the e-mail account and create a message, so I'm happy with that app.
I was also checking out MailDroid. It has a lot of the same features as K-9 Mail, and is supposed to match up as a nice GMail client replacement, so it could be a good app for all accounts. K-9 Mail has the ability to print email (through PrinterShare - non-HTML messages), so I've chosen that app for now. MailDroid keeps getting some very nice updates, and pretty quickly as well.
I haven't tried Gmail through K-9 Mail, but so far, having the two apps for email has worked well for me.
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Some of the tweaks I've done to my Mesmerize have helped with the transition from BlackBerry to Android:
- Added the Bedside app to give my SGS bedside mode.
- Added Quick Profiles so I can switch profile settings quickly.
- Added NoLED so I have notifications dots on my screen (since the SGS doesn't have a notification LED). I even have different colors for each email account.