Offline Use of Sparrow
I use Sparrow daily, but haven't ever needed to use it offline. From a few different websites (after Googling), it appears as though people do use it offline, but that it is still slightly buggy. I found a blog post that talks a bit about how it works great offline. There was also this question over on webapps that suggests using Sparrow or Mail.app (if not the offline Gmail feature).

It should sync up fine since it's just another IMAP email client, but as usual, be prepared for the slight possibility of some issues arising.