Working while in school is the best thing I've done for my career. Right out of college I had 5 years of experience, which is extremely important for employers. So after I graduated I got jobs that my classmates could only dream of.
I also wish I'd had the opportunity to spend more time with my fellow students. Not only for the friendships that might have developed, but for the intellectual development that would have come from discussing our coursework or just the world in general.
I always assumed I have plenty stimulating discussions with knowledgeable people once I entered the workplace, but sadly that has rarely been the case. Maybe I need to get out of Enterprise IT. :-)