If you are mining for fun and education, then go for it.
If you are mining for a profit, then you really need to do your research. Because of the expected rapid rise of the difficulty, the rate at which a device will mine bitcoins will drop very quickly. At some point, the amount of bitcoins that it mines each day will become negligible. My estimate is that each 1 GH/s will mine about 0.05 BTC over the next year. If you want to break even with a USB block erupter, you will have to pay less than 0.017 BTC for it.
yes but at the same time its the cheapest device out there and aslong as your willing to wait then you will make a ROI eventually
vm1990, you haven't done the math. I suggest that you sit down and figure out how long it will take. Depending on how much you pay for the mining equipment, "eventually" could means decades or centuries, or longer.