Gmail, Google Apps, iCloud, Yahoo, AOL and custom email addresses are fine to use here, but there's no support for POP or Push notifications just yet. The latter is apt to put off the power users, but more casual readers may appreciate the unified inbox, direct-attachment feature and plenty of other details that provide for an overall delightful experience. It's available now in the source link (or the App Store, if you're up for a search), and we're told that localization, landscape mode, a built-in browser and 'Send and Archive' will be coming in future builds.
Psst... hit the More Coverage link to read Sparrow's explanation of why Push is being left out. If you couldn't guess, it's about privacy.