The amount of hashing power doesn't determine whether it's a good buy or not. What matters is hashing power per BTC and to a lesser extent hashing power per watt.
In other words: Price matters. A USB Block Erupter would be a very good buy if it could be bought at 1 mBTC, despite it only having 330 MHash/s.
This.
No. Even if I give you a 333 Mh/s USB for free it makes such a very insignificant amount of coin its not even worth it unless its just a hobby. I and others want to make money and with a few 100 Gh/s you can do that. If you want to see how much people are making look at the pool payouts. Right now even people with 50 Gh/s are making a good income.
Do not under estimate 1 block erupter.
When I start mining in june 2011, bitcoin price fall from 30+ dollar to 10 dollar after a rally.
Bitcoin keeps falling, after a while it was only about 2 dollar a coin.
With only 5 coins a day I was only making around 10 dollar a day.
Then some people start move to altcoins.
People who stay to mine Bitcoins get even more coins a day.
Most of those premined altcoins already died nowadays.
I keep adding hasrate.
Last year I went to The Efteling (A dutch version of Disneyland) with my daughter and bitcoin start to rise again.
It was then 7 dollar in juli 2012.
I said to my daughter, your computers already earn you an amount with 5 digit.
Everything my miners earn is for my daughter, I started to mine with her broken HP G72. The screen was broken.
And then I took everything I could re-use from that HP to build a new desktop with an Asus 6870 graphic card, soon I was adding more GPU's and keep building more computers and better GPU's.
What 1 block erupter will earn you nowadays is nothing, but if you understand how Bitcoin works, you will know that it will be worth a lot after 1 year.
So everything what you mined today, spend that after at least 1 year and save some.
You probably don't know the story of the guy who order a pizza with bitcoin for 10000 bitcoins.