I hate E-mail. Work- flow gets drowned out by a huge amount of noise from idle chat, marketing notices, newsletters, and spam. I have three E-mail apps on my Droid. I use all three. Each E-mail app has their strengths and weaknesses.
Yes, I can survive on the default Gmail app but it's not sexy and the configuration options doesn't really allow me to get to the significance of the content quickly. Gmail is fast, simple, and has a light footprint.
K-9 Mail has more options -- some say too many. K-9 has a small footprint, many display settings, and a content sharing ability that I use heavily. Once the initial setup is satisfactory, it is the preferred work-flow app.
MailDroid has a sexy look about it, and the spell checker is appreciated. Nevertheless, it has the biggest foot print of the three packages, and does not share its contents easily with my work-flow. Note that I'm always stressed for space on my phone. And I hate ads.
Until something significantly better comes along, K-9 Mail gets my best E-mail app vote.