Is it worthy to use "bitcoin usb block erupter" I was searching for Mining hardware on internet and found that a new technology is available known as "bitcoin usb block erupter" for mining bitcoins as 300MH/S

I just want to know if it is useful to use these hardware instead of GPU mining and how to calculate breakeven point for them.

P.S. : I've searched google for many things but there were mixed responses that's why I asked this question here
A typical USB block erupter will get 333MH/s under realistic conditions. Today, a share is worth about 1/156 of a penny and 333MH/s will get you a share every 13 seconds. That comes out to 43 cents per day per erupter, not counting the cost of electricity. Difficulty is going up, and thus profitability going down, around 25% per month. So you can figure maybe $11 your first month, $8 your next, $6 the next, then $5, and so on. You might get about $50 before it becomes more trouble than it's worth.