Sorry for the cross posting from another thread but I happened to notice this thread and thought others might benefit from the comments. Note that several of the issues touched on here have been mentioned in other posts on this thread, specifically the issue of whether to delete or not delete email from your ISP's server. Just thought the following discussion might provide a concrete example of what happens with various configurations.
Managing email can be very complicated because it depends upon a number of variables that differ according to your ISP, possibly the source of email (e.g. Exchange, Yahoo!, etc.) and the settings you create for email forwarding and deletion of messages on the server that provides your email.
I use both gmail and K9 in conjunction with an Exchange server (for corporate email), Comcast as my principal email server, and Outlook 2007 on my desktop computer. No problems. My wife's Droid has a similar setup except that she doesn't have to deal with Exchange and has Outlook 2003 on her desktop system. It's been an interesting (and sometimes frustrating) experience setting it all up.
First, why do I use both K9 and Gmail? Simple. Gmail's "threaded" presentation of emails is useful in some situations and frustrating in others. K9's more conventional separation of emails can be useful in some instances. Also, as noted in the posts above, it is sometimes preferable to reply to an email from the address to which the original email has been sent.
I don't use the Droid "email" or "corporate email" processes at all. For my purposes they're redundant.
Here are some points to keep in mind.
() If you use Exchange have it set up to forward your email to your ISP (if it's different) and delete it from the Exchange server. This prevents you from having to "clear out" your Exchange email account. (Note: Not critical and you may not be able to have your Exchange administrator agree to do this, but if you do so, it will remove Exchange issues from the entire configuration.)
() If your ISP allows you to do so, forward all email to gmail FROM THE ISP's SERVER and NOT TO DELETE it from the server when it does so. This assures that you'll get all of your email at the Gmail server. It makes an excellent archive for email. I've used it for five years; I have 30,000 emails on my gmail account and use only about 20% of the free space provided by Google.
If you do this correctly, you won't have to worry about accidentally deleting an email or having K9 not be able to find an email when you're ready to read it on your phone. The email will always be in your gmail account where you can access (or delete) it.
() If you use an email client on your desktop system (like Outlook or Thunderbird), decide if you want to delete an email on the server from that client application. If you do (as I do) recognize that K9 (or the droid's generic email application) may tell you there's an email on the server but by the time you try to look at it, the email cannot be found because you've already deleted it.
() Set up your K9 account to poll frequently if you want to be notified that an email has arrived. Or just rely on the notification from gmail. When you open K9, it will either be there, will be there shortly, or will be downloaded when you open K9.
() Decide if you want to be able to delete emails ON THE ISP's server from your K9 account. If you do, recognize that if you also allow deletion from your desktop system you may get an error from one client or the other when it tries to read an email you've already deleted via the other client. (Essentially this is the same issue noted above in reference to deleting from your desktop client.)
() If you want to keep track of all your replies, consider setting up a "rule" or manually inserting a BCC to your gmail account whenever you write an email (perhaps limiting it to particular addresses.) I do this to assure that all my replies end up in my gmail account even if I reply from my desktop Outlook client. It creates some redundant emails in my gmail account but gmail is so good at searching for emails that I can always locate a reply to a particular email regardless of which client I use to respond. (Note: I have to do this manually on K9; there isn't (as far as I know) a way to automate a BCC when I reply there. YMMV.)
Sorry for such a long-winded commentary but hopefully it will help others who have to sort out complicated email configurations. Probably the most important point in all this is to be sure that DELETING email from the ISP Server is set up as you want it to operate. All sorts of anomalies can occur if you either set up deletion from a client and then try to access the email from another client.
I strongly suspect that some of the problems people report about gmail, K9, and the generic email app on the droid are actually associated with an incorrect configuration for email with their ISP's.