We've been square foot gardening for several years. Initially it was a lot of work and money (although my dh got most of the wood from the cull pile at Home Depot, which saved us a lot). Our boxes are 8" deep. Once we made the Mel's mix, it's so easy now--each spring my dh empties the boxes onto a tarp, we fill the box halfway w/compost, and cover w/Mel's mix. We started the boxes this way, too (bottom half compost, top half Mel's mix).
So each year we add in more compost, and in my mind anyways, since all the soil is getting mixed up, it's similar to rotation planting. Most often, the compost we use isn't totally composted yet, but by the end of summer it is.
We've also done a variation on this method using buckets w/drainage holes. I didn't like the idea of plants that take up more room, like tomatoes and cucumbers, being in my precious boxes. So in a large area of our yard we dug holes to put the buckets in, use the compost/Mel's mix as above, and have those plants in them. This way we didn't have to create a huge area of good soil. This year my dh added a large trellis along the back, so we'll see how that works for things like squash and cucumbers. And this will be the first time planting tomatoes in them, so I'm curious to see if my tomato plants are more manageable since they'll be further apart.
I think Mel's mix is one of the best ideas ever. When we recently came back from vacation (we hadn't planted anything yet, but our boxes were ready), they were covered w/weeds. No problem--they easily pulled out without any hassle.