In my mind, I ambitiously call my middlebuster a plow. It's also a plow when non-tractor folk ask what it does. Technically, of course, the similarities it shares with a real plow blade are offset by the differences. But, to me it's still a plow. At least that's the way I use it. I run it as deep as it will go, to turn the garden soil in preparation for planting in Spring. Sort of a poor man's tiller. I smooth the dirt back out with boxblade or loader bucket before planting. Then, again in the Fall it's used to turn under whatever is left on the surface. It works great. Other uses as described by Bo, above.
After following threads here on TBN I feel pretty comfortable about knowing what a subsoiler can do. I've never used one, so I won't bore you with my thoughts. But, I would like to have one. There're plenty of TBNers who can give you first-hand feedback. Also, you might try running a search.
OkieG