Though the Android OS includes a Gmail application that gives you easy access to Webmail, K-9 Mail is one of the few free e-mail applications in the Android market that provides access to POP3 and IMAP e-mail accounts. Unfortunately, while the program works as advertised, it runs extremely slowly, especially when downloading dozens of messages from heavily used inboxes.
In my case, I get a lot of e-mail. When I tested K-9 on my very busy POP3 account, it was able to pull down the message headers and generate a list of messages, but only with an extended delay. On a number of occasions the program seemed to stop responding during a mail download, but was probably just processing the large number of incoming messages.
When the program seized up, I found that I could simply force the application to quit (I did this by switching back to my home screen, launching TasKiller, and killing the app's process); relaunch the app; and once again have it scan for new mail. That's not a pretty process, but eventually it worked. I just wish the application didn't have such problems with large batches of incoming mail.
I'll continue to use K-9 for my POP3 email--after all, there aren't many other choices if you don't want to use Gmail, and you don't have a corporate Exchange server--but it has room for improvement.