I know that this will annoy some people, but here's a video of the 37signals guys kissing Apple's ass:
http://www.apple.com/education/whymac/compsci/video.html
Forget about the content... look at the office. It's reflective of their Web apps. It's sparse, functional and simple.
I'll admit that I'm sick of hearing about these guys, but why am I so drawn to what they have to say? Why did I buy the Getting Real e-book? Sometimes I'm annoyed at myself for being into their world.
I think to answer this obsession question, we have to look at the two sides to the life of a developer. On the user side, simple is better in most cases. Who uses more than a fraction of the features in Word, right? I just want to write a damn letter! So the fact that Backpack or Ta-DaList does something fundamental in a fundamental way allows me to "just do it." You can't beat that appeal.
From the programmer view, they do what many of us only dream about. They build stuff quickly, it works, it delivers, it generates income for a company that consists of less than a dozen people and we're jealous. How hilarious is it that we all want to build The Next Big Thing and have it used by giant companies (or have your company bought by said companies), and these guys build something simple for the millions of small customers that don't buy products from IBM or Oracle.
I hate those guys because I'm not them!