I've had a 4-day work week (working 80%) for a while and it's great. If your circumstances allow you to do this (and I recognize that most people can't), I strongly recommend it. I've been able to do a lot more on side projects (like blog posts). It also gives me much better "work-life balance" and more time to spend with my family - I know it's a cliche, but kids will be gone before you know it so spend time with them while you can.

A few disadvantages: First, I only get paid 80% as much. Second, it took a fair bit of effort at work to set this up at work; management was supportive, but they'd rather I worked 100% and it took multiple levels of approval. Third, this basically puts me off the career fast track. Fourth, from a social/cultural point of view, people don't really understand working 80% and I find it's easier not telling people about it (thus the throwaway).