Its a tough question - I think it varies for each individual. I worked with FedEx part time on a safety project while doing my Masters - my whole thesis was evaulating and improving their truck safety operations. That required me to come in and work with them on a 9 to 5 basis daily.
Some schools look upon an MS as an additional work experience. Some don't, but I think through your essays, you have to show what valuable experiences you have learnt and what skills you have developed.
I developed more leadership and "crowd control" skills while teaching a bunch of undergraduates, than by working with my peers in my office.
Also, I worked with huge data sets, made financial models, used probability distributions in school. At office, I am an analyst and the only thing I have done is analyze the different fonts in Excel.
End of the day, its how you put your experiences in your application- you may have worked 10 years and learnt nothing, but you may have worked 1 year and learnt a lot.