That depends on how the customization is done. There are customized maps that are loaded with good starting positions for everyone, but the assumption is that everyone will be at war shortly. Then there are customized maps where the starting positions are so-so, but all of the goodies are surrounded by hordes of barbarians. Probably the best way to do it is generate a map, and then do a bit of customizing yourself, to fit it to what you are looking for. The best scenario map that I have found so far is the Age of Discovery scenario in Civ3Complete, and one of the best customized maps I have found is the Turkhan series in the Civ3Complete Content selection. Some of the other ones are pretty good but boobytrapped in some way or another. A good short game is the Dinobarbs one, with good starting locations.