For me the impressive part is that they've buckled down, slowly evolved the core in a way to add features without upsetting their existing happy customers to an extent that there's been much screaming online, and released additional applications along the same template that have also been well received.
Basecamp provided them with product/market fit - iterating on a successful business model in such a way as to gradually grow a profitable company might mean that they should no longer be considered a startup, but it's no less impressive.
Perhaps more importantly, for those of us wanting to build a company we enjoy working in that isn't trying to either (a) scale massively (b) sell (c) IPO they're a great reminder that such stories do exist.
Plus I think I rather enjoy the sheer chutzpah of their shameless self promotion, even if I do like to think that I've not entirely drunk the koolaid.
Basecamp provided them with product/market fit - iterating on a successful business model in such a way as to gradually grow a profitable company might mean that they should no longer be considered a startup, but it's no less impressive.
Perhaps more importantly, for those of us wanting to build a company we enjoy working in that isn't trying to either (a) scale massively (b) sell (c) IPO they're a great reminder that such stories do exist.
Plus I think I rather enjoy the sheer chutzpah of their shameless self promotion, even if I do like to think that I've not entirely drunk the koolaid.