I use Sparrow because (1) it's zippier than Mail.app (2) shows unread message count in menu bar and most importantly, (3) I can drop an attachment and it creates a dropbox URL automatically (or not if you wish).
I wish I could hit "send" before it had finished uploading to dropbox - it could have uploaded in the background, created the dropbox URL and sent the message silently when everything was ready. As it is, I have to wait. Other than that, Sparrow works great.
I don't use any archiving or folder/categorizing features (my "Inbox Infinity" approach to email management
so there's no lock-in to Sparrow.
I bought it for $10 a while back. Since it's no longer in development, Google ought to make it free methinks.