The fancy tailed guppies are without a doubt the prettiest of all fresh water fish. The only problem I have with them (and I have been raising them for 10 years) is you should not put more than 2 males together in a tank smaller than a 20 gallon unless you want their tails to be nipped at and torn.
I have kept Guppies on and off for 20 years or so and love them. They are ideal pets, incredibly enough showing signs of social structure and bonding with the owners, rather like my 6 cats do! Though I am branching out into other kinds of fish and eventually plan on a marine tank, I will always keep a tank or two just for the "colorful friends". I highly recommend them for their ease in raising and also for their temperament.
I just bought some feeder guppies for my 2 Jack Dempseys. I keep my guppies in a separate tank, and I have bought some fancy male guppies so that my tank will have more color to it (seeing as how most feeder guppies are drab in color). My tip is, when you clean your tank, take out only 1/3 of the water or you will find dead guppies in your tank the next morning! (Especially the fancy guppies!)
I have been keeping Guppies for a year or so and I found out that the cheap feeder guppies are a lot smaller than the fancy guppies. One reason could be that when guppies grow they release a stunting chemical into the water and if the water is not changed (which it is most likely not for something as cheap as a feeder guppy) then the guppies don't reach their full size. This would help them in the wild because if there was a small pool, and guppies somehow got in it, then they would survive better since smaller fish need less food.
In the past few months my fancy guppies had several batches of fry. The two things I recommend to keep them alive is remove the fish that eat the fry, and to feed them baby guppy food, they love it. You can find it at most majior pet stores. The guppies are not hard to breed, in fact they are hard to not breed. If you have a male and a female in a tank they probably have already bred.
To breed these fish, I recomend having some java moss! The fry love to be in it! All you have to do is tie a small amount of java moss onto a log. The fry like to be in it because they get protection and food gets caught in it sometimes, so they don't have to come out. I just started a 40 liter tank and there are 2 females and one male fancy guppy. I brought the females when they were already pregnant. Java moss and a good log will make a good pad for guppy fry.