I got myself one of these some time ago. But so far I haven't done much with it. It seems to me though that with some additional hard- and software (like additional sensors and a "real" programming language like "Not Quite C" ) it's more fun?
This is one of the two sensors I built, using a "Slotted Opto-Switch". I got this from Malc's webpages [unfortunately the links seems to be broken by now]. Counting wheel revolutions or sensing an "end state" of some movement can be done with it. Malc has other interesting sensors to offer and there's more on the internet. I want to build an "inclinometer" some time.
Also I bought two "Droid Developer Kit's" with the "Micro Scout" from Lego. Its got a microphone and a light sensor and just one motor. So, what I said above applies here too: This stuff has GOT to be hacked to be useful ... :)
What I would also like to do is to convert some standard robotics servo motors so
that they are Mindstorm compatible. They would have to be connectible to Lego aisles
and so on.
Of course, meanwhile (some years later ...) there's 'Mindstorm Next' ... (no, not yet)