I have worked with school children on Mindstorms projects (for First Lego League) for a number of years now, I also have 20 years experience of running professional engineering design teams.
The more I work with Mindstorms, the more impressed I am with it. The original Mindstorms certainly had bugs, but they don't stop the children a) picking up its use quickly (very important) and b) then taking it on to very complex levels. I have recently recommended it to my local university for undergraduate robotics introduction, equally I know someone who uses it with infant classes!
In terms of real world skills, it does allow structured programming techniques, and certainly encourages problem solving. I haven't yet looked at NXT, but to make the original system work well then the use of feedback in the system was essential (otherwise your machine performs differently as the batteries run down!) which again teaches valuable lessons about engineering - for it to work reliably you have to allow for and compensate for its limitations.
I was fortunate enough to attend last year's national finals of the First Lego League, the standard of programming and engineering shown by the finalist and semi-finalist teams was incredible - these GCSE level school children had solved problems which I couldn't work out how to solve.
The major issue I have come across is pure snobbery from robotics lecturers over the use of a 'toy' - luckily I don't think students care. Oh, and if it was half the price that would be nice too!
Hope this helps, please feel free to use any part.
Schools Liaison Officer, IET Devon and Cornwall
Engineering Design Manager, Bombardier Transportation
Andy Millar CEng MIET CMgr MCMIhttp://www.linkedin.com/in/millarandy
"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert