Quite simple, really.
It's pretty much as powerfull as a high-end ATI GPU, but at a fraction of the power requirement.
An ATI HD7950 will set you back around $300 (or €260), will consume around 250-300watts of power and give you 360-380Mh/s.

An USB Block Erupter will set you back 0.5BTC (roughly €40 or $50, see forums for groupbuy), consume 2.5watts of power and give you 330Mh/s.

In short: for the price of one 7950 you can get 4 USB-BE's, so thats 4 times cheaper already. Effectively, you get 3,75 times more hashing power for the same amount of money.
Then comes the power requirement. You could virtually power these things from pedaling a bicycle an hour a day. (just put an alternator on, hook up a large battery to store power and control the BE's with a RaspberryPi or a modified router running Linux)
They really won't increase the power bill. A single 7950 will cost you anywhere between $10 and $30 per month to run 24/7 (depending on where you live), wich at the current exchange rate (BTC->USD) will never pay for itself.