We walked around with a Digital Guide (at an extra charge) which played videos and commentary as you walked around. I wouldn't say it's a must have to be honest as I felt like we didn't really get to share the experience together, we were all walking around with headphones on listening to the guide throughout the whole thing. What they tell you in the guide is on information boards around the tour as well so I probably wouldn't bother with a Digital Guide. Unless your an avid fan and want to see small interviews with the production team ect.
I honestly can't put into words how incredible it was to see how they made these amazing films! The detail that goes into every little thing is mind blowing! Even if you're not a Potter fan (what the hell is wrong with you if you're not?) I seriously recommend going along to the Harry Potter Studio Tours.
It definitely didn't ruin the magic of the films either which is what I was kind of expecting, if anything, it makes you appreciate them even more because you know how much effort went into making that washing up brush magically clean the dirty frying pan in the sink ;)