That speaks volumes about Sparrow as the app costs $9.99 while the web-based access to Gmail, the only email currently supported by the app, is of course free.
For those using Gmail who've been looking for a clean and simple native Mac client, then Sparrow might be just the thing for you. Honestly, I'd say that even for those who haven't been looking for a desktop client.
The common refrain you'll hear about Sparrow is that it's like Twitter for email. Indeed, the interface is very reminiscent of Tweetie for Mac. There are three columns: the left column contains your inbox, starred, sent, drafts and trash. The center column contains a short preview of messages. And the right column, which is collapsible, contains the full email message.
Sparrow supports many features from Gmail, including labels and, should you choose, Gmail shortcut keys. This makes the transition to Sparrow easy, whether you're a "hardcore" Gmail user or not. The interface is very intuitive. Arrow down, for example, to get to the next message. Arrow right to open the far right pane, and arrow left to close it again.
Sparrow's "quick reply" feature is great, and keeps with the app's minimalist interface. When you hit the reply button in message views, you'll get a small text box, ideal for typing quick replies to simple questions. But you can also pull up more options with the reply button's dropdown menu, including the standard reply, repl all, forward and delete options.
The app also supports Growl notifications, allows you to drag-and-drop images directly into messages, and displays your messages in threads. The one feature that I noticed that's missing is "mark all items as read," but I'm not sure you're supposed to publicly disclose that you do such things to email.
And while we're on the subject of disclosure, I'll admit that I was pretty skeptical initially that a desktop client could woo me away from the web. But having taken Sparrow for a spin, I'm impressed. And now, I'm a user.
Currently, Sparrow only works with Gmail. You can add multiple accounts - great for those who have personal Gmail accounts but also use Google Apps. As Sparrow uses IMAP, support for other email services will is in the works, with Version 1.1 out soon.