Ski resorts lie about their snow coverage, and tech vendors lie about performance. Marketing, for example, convinced most geeks that USB 2 was as fast as Firewire, but that's certainly not true under OS X.
In the same vein, here are two articles that contrast real world performance with marketing claims:
Coding Horror Gigabit Ethernet and Back of the Envelope Calculations: A 2005 article that showed Gigabit ethernet is about 3 times faster than 100 mbps ethernet -- not 10 times as fast. Great discussion.
Dans Data review: USB 3 drive box and controller card kit (Jan 2010): In most available machines USB 3 is 3 times faster than USB 2. (So it's probably faster than Firewire 400 and maybe comparable to Firewire 800).
Threefold improvements are nothing to sneeze at, especially in the post-Moore's Law era. It's just not quite what marketing promises.
Mac users need not feel USB 3 envy -- we get Firewire 800.