Boomerang started as a get stuff in your inbox when it's relevant tool and that's been the biggest win for me. But the scheduled sending is quickly rooting itself into my email workflow. I am almost always heads down coding late at night and will often take breaks to wade through some email. Replying to everything at 3am is not ideal as I then get caught in the 4 - 8am slam of crap email that fills everyone's inbox. Choosing to have my stuff hit an inbox at 10 or 11am, so I'm after the crap and before lunch rocks. Wait, did I just give up one of my super secret competitive advantages over all the rest of you startup folks?
Anyway, I've been using Boomerang since early in their alpha. I've also know the Baydin team really well (we went through TS together in 2009) for a while. Good guys working really hard to solve real problems and make email not suck.
Is it perfect? Not yet. It requires a new version of FF, the Boomerang button is on the left of the Save Now button when you're composing (change this please Alex!), and it can't reply to my email for me, but so far it's pretty freaking awesome. Inbox 0, Tom 1.