I have 2 USB miner's and i hit around 660mhash/s give or take. I've made about 0.02361080 in 2 weeks of almost 24/7 mining (got kicked off the internet) with BTC guild. I paid around $18 each and I really don't expect to make my money back. If you're looking for a quick ROI or even a ROI then avoid these. If you just want a novelty item and to claim you're a 'member' of the network then they're worth every penny.
The same sort of calculations apply to all new hardware don't they? Most people seem to labour under the delusion their shiny new piece of next generation hardware will be the only one in existence. Over time the magical ROI number will stretch further away into the future and an investment break even point impossible, all the while running costs increase as a percentage of returns and also compounded as, in absolute terms, energy prices rise over time.
For a hobby yes, anything else I'd have to say forget it you're on a hiding to nothing.
