I did this too - a part-time PhD with a full-time s/w engg job. So its good to know there are more crazy folks like me. Essentially I took up my PhD in a topic that was statistical as opposed to what I was working on distributed systems/applications.

It greatly helped broaden my horizons professionally. The writing experience you get doing journal & conference papers is excellent. Your analytical & critical thinking skills will get an immense boost and you will know how to critique, publish & communicate well. And when it is all done you will understand why some call a "worthless academic adventure" is one of the best things you should have undertaken years ago.

To anyone on the sidelines - do it. Don't think, do it - there will be times when you think you can't and just have to quit. But have someone around who can support and kick u back in. I have an awesome wife who pitched for me every step of the way... Also have someone to provide you with the motivation to finish. I had an impatient 3 yr old :)