I commissioned in 2006 and went to AFIT for a year of grad school. My experience has been very similar to what Pags was describing. Granted, I'm still in my first fleet tour. I would say I am roughly 18 months or so behind the people I graduated from school with in regards to quals, experience, and time in the squadron. I'm going to have to find ways to make up for the time I lost. I'm currently looking at rolling from my squadron a couple months early (mostly due to the transition to the P-8) to get some of that time back.
With that being said, if you are given the opportunity to get your grad degree: DO IT! I don't regret my decision one bit. I had a blast that year in grad school and I think it made a difference for me in flight school. Not because I had that grad degree, but because I was one year older and just a bit more mature. I knew how to manage my time well and how to focus on the task at hand, which comes in very handy through out flight school and your Navy career. Grad school only further reinforced the importance of those things.
One caveat, after all of that work I did on my Master's degree, I have yet to use it. I'm a very glad I have and I learned a lot from my grad school experience besides the technical knowledge that a Master's degree teaches. However, nothing I do now requires a grad degree. After all of the work I did, I'm starting to think about ways I could use it in the Navy. It will open other opportunities up to me down the road (AEDO or TPS as examples). So, take advantage of the opportunity whle you can.
One final thought... Take advantage of every good deal in the Navy you can get. I say this because the Navy will find a way to get you to 'repay' that good deal down the road.