I bought Sparrow and have come to prefer it over Mail.app. Here are three reasons why.
1. Quick Replies
I hit R on a message and I can instantly reply without opening a new window. It makes Sparrow feel really lightweight.
2. Better Gmail implementation
Mail.app, even in the Lion update, feels weird with Gmail. I don’t like archiving everything. I want to be able to delete useless emails. Using Gmail with Mail always gave me some archive anxiety. (Did I delete this message or archive it? I better go to Gmail.com and double check.) I thought that the new Archive button would fix that behavior, but instead it creates an Archive email folder and moves email into that. That’s great for IMAP and Exchange accounts, but doesn’t fit in with Gmail’s All Mail system.
Labeling (and, I assume, moving to folders) is also easier in Sparrow. Just hit the L key and a label selection box appears, just like in Gmail.
Attachments in Mail.app are pretty horrible. Attachments in plain text emails appear inline. In Sparrow there’s a dedicated attachments area that segregates your message and your attachment.