Mail.app does not play well with gmail. Sparrow was bought by Google (and therefore has uncertain future on OS X. Inky seems nice, but it's terribly slow and is a resource monster. So what do you use for your email?
I'm using MailMate[1], which is a brilliant client developed by the Danish coder Benny Kjaer Nielsen. It has too many neat features to name, but the one I absolutely couldn't live without is the ability to write emails in Markdown.
I need email for work. If I cannot be sure about my mail client I must change client. However, Mail.app never satisfied me 100%, so I took the opportunity to scout for better clients.
I started using Airmail[1], and I love it. Mainly for label support with Gmail, but there's lots of little bits of UI features that I love about. Definitely worth the $1.99 or whatever it is on the App Store.
I use mutt[1] for all of my e-mail. It's a bit of a bear to set up intially, but really worth it. There are lots of guides online that ease the pain of setting it up.
Have migrated over to Outlook.com as well and can confirm that the user interface is quite refreshing when compared to the deteriorating state that is gmail. Be aware that Outlook.com is not 365 and there is unfortunately no paid or premium support option, however entire domains are supported via going to domains.live.com. Perfect for family emails/domains, etc.
After migrating I authored a plugin for Google Chrome that makes Outlook.com your default email application and provides a button to compose a message to quickly share a link via email. https://chrome.google.com/webstore/detail/send-from-outlookc... if your using Chrome + Outlook.com check it out.
I don't like the interface. I find it too clumsy with features I don't use. Said this, at the moment I'm using it :-(. I used Gmelius to get rid of most of the interface and it is usable. However I'd like a better integration with the operating system.
[1]: http://freron.com/