I'm just a little over a year in and run all 8 frame medium equipment, a decision that I've been very happy with. But in all fairness, I really have nothing to compare it too. I think there are a couple advantages and disadvantages concerning 8 frame medium equipment which are unarguable.
Advantage: Weight savings, making the task of having to lift and move boxes a lot easier and pleasant task.
Disadvantage: Drives up equipment cost...Both more frames and boxes are required to accommodate the same number of bees.
That being said, I do believe it makes it easier for them to fill out the end frames. But there are 10 framers who will argue they have no problem, or their problem is "simply" solved by moving these frames inward.
I've also heard heard numerous claims of of the things 8 frame mediums can't do, or don't do as well...but this seems to be mostly hearsay.
Have you read what Michael Bush has to say on this topic? He's got quite a bit of experience and is a pretty big advocate for 8 frame mediums. Here's a link to his site, where his entire book is available!