I currently have two very small garden plots in a dirt lot next to my relatives' house. One, I double-dug and built a gopher barrier for and mixed compost into the heavy clay soil, and planted some basic things in a square foot gardening configuration -- green beans, radishes, watermelon, cucumber, etc. But there's a lot of clay and the sun is hot, and though I have a few sprouts, not everything came up, and those that did come up are slow and stunted.
A week later I bought a bag of potting soil from home depot, didn't dig at all but just spread it out about an inch thick (though very unevenly) over bare hard ground, threw some seeds on it randomly, and covered it lightly with some dead leaves. Though it's been invaded by slugs (I guess they like leaves), it's sprouted quite abundantly and the sprouts look healthy! Some are nibbled, but not destroyed. There are so many bugs and so much life in there.
Goes to show more effort is not necessarily better.
Also goes to show that clay is evil.