I've used Johnny's Seeds field peas/oats combo in early September for a late autumn till in my community garden plots - but only on my plots. I think it's helped a lot, I can tell the difference between my soil and the soil of the new plot I just took over this year.
I'm going to experiment this year with some summer cover crops (buckwheat or rye) in my spring greens/fall greens plots, and I'm going to try early spring cover crops to be green manure.