Couldn't disagree more. I switched to Twitter web and Gmail from desktop counterparts several months ago, and haven't looked back since.
I hope demos are a feature they're working on.
Seems they're working on a non-Mac App Store version too. http://blog.sparrowmailapp.com/post/3197243085/sparrow-1-0-i...
It's not cheap, but it's well worth the price (its also due for a paid upgrade soon, IMO, as I have not paid anything beyond the first version three years ago).