The problem with espresso that doesn't come from a good machine is heat. The water needs to be between 190-195 F for the proper extraction without the nasty by-products.
Extraction temperature depends on the particular beans that you're using. Some beans call for higher extraction temps. For example, the roaster of Malabar Gold stuff I bought recently recommends using 202-205F.
General rule of thumb is lower temperatures for darker roasts and higher temperatures for lighter roasts. And sometimes you just have to experiment and see what you like best.