What do you like best?
I first downloaded K-9 Mail when my Android's pre-installed e-mail client wouldn't work, and have been using it for everything except my personal G-mail ever since. The app is simple with very little in the way of frills, and it just works.
What do you dislike?
The biggest annoyance is trying to search for e-mails that have since been deleted locally. Currently, only e-mails that are on the device are searchable, so if you want to search beyond the 50 or so still stored locally, you have do download past e-mails, 50 at a time, which then show up as unread. If there was a way for the app to search the server and only download e-mails matching he search criteria, the search function would be much more useful.
Recommendations to others considering the product
If you're looking for something with a lot of features, look elsewhere. If you want an app that is simple and just works, then this is it
What business problems are you solving with the product? What benefits have you realized?
K-9 lets you combine multiple e-mail accounts into one, easy to use, highly intuative client, and just works when others don't