...as much as I kinda like a lot about 37 Signals work, this sounds a lot like fishing for sunk cost: "I paid 99 bucks for that, I have to use it or I wasted money." It's not an uncommon practice, but it's shady.

The product sounds nice, especially the thread grouping. I might even go for it once they have personal domain support, though I'm rather wary of the difference between what they try to do, and how it will work in practice, and pricing it at double of, say, Fastmail, makes me feel like someone's trying to exploit me.