When I installed a greenhouse five years ago, for the first time I had to ability to start a large number of plants early. I was so excited I started everything early in the greenhouse with the idea that it is always best to get a head start on the natural limits of cold weather and soil temperature. I've learned that this isn't true for every plant in the garden. Even if you don't have a greenhouse you will soon be faced with store shelves and farmers' market tables loaded with lush green plant starts that are hard to resist. The immediate satisfaction of a real plant is certainly more fun to buy than a pack of seeds but like most things in life, delayed gratification is one of the key lessons of gardening.
I recommend buying and growing plant starts for: