The general consensus is that if you don't have an order in already, by the time you receive your unit, 5Gh/s will be too slow considering where the difficulty will be at that point. The ROI would take too long to make it worthwhile. If you think the price of BTC will go up, it's better to buy the coins and hold them.
It all depends on how much you're paying for electricity (and what the USD/BTC exchange rate does in the future).
Even with a total network speed of 5 petahashes/sec, the 5GH miner will be producing about 0.11 BTC/month. At current rates, that's around USD$15. It would still pay for itself in 18 months (plus a few extra months for shipping).
Obviously if the exchange rate takes a dive that $15 could very easily become $1.50/month.
I'm not really worried though. In the long term, Bitcoin will almost certainly appreciate well past the current US$130 price.