FWIW, I have been using Mailplane on Lion on my MacBooks (Air&Pro) without any bugginess. (Have you contacted the dev? He's been very responsive in my experience.)
If you didn't do a clean install of Lion (or used MobileMe to sync preferences) you might want to backup
- ${HOME}/Library/Application Support/Mailplane
- ${HOME}/Library/Preferences/com.mailplaneapp.Mailplane.plist
(If you don't have your license file, be sure to save
defaults read com.mailplaneapp.Mailplane License
too.)
zip/email them to yourself and then delete them -- not just trash them -- from your system. Reboot.
Then try setting up Mailplane again.