I started working with it last week, now that it's updated and on the app store. I'm impressed. I've been a dedicated Apple Mail user for years, but I really like Sparrow's take on things. It's more of the true "Gmail" experience that I know is hard to adopt to for some. Only now that I've really given it a shot with Sparrow do I think it's going to work out for me. I'm dying to get away from the old protocol of folders/subfolders and all the time wasted "organizing" mail I really don't need to access much. My old behaviors are still deeply ingrained, but Sparrow is finally helping me with the discipline I need to re-think email.
Sparrow still has a way to go, but I've recently had good experiences with similar upstart apps that have agile development in their favor. Looks like v1.1 is expected to have full support for non-Gmail accounts. I still need that to do a full switchover, but non-Gmail accounts are less than 5% of my overall mail experience.
I'm not 100% into it, YET, but I think I'll be there soon. Need to really re-train myself about the Google approach to email, labels and search as opposed to filing, but I'm sooooo wanting to get there.