Realizing I'm very tardy to this post, I'd like to throw out this update for those currently googling this question.
I purchased 4 erupters back in November 2013- just to run beside my 4x5970s and 3x7950s, it was an experiment and at the time I was pretty new to mining in general.
4-erupters with 92mm cooling fan and powered-USB hub. I've had a constant 1.3-1.4Gh/s with these USB erupters, peaking at 2.2Gh/s (but that's an anomaly and not constant) I use bfgminer and set them up to run @bitminter. They're super easy to use once you get the drivers set up and understand how to mine. I chose mining @bitminter because it seamlessly allows you to mine namecoin through the same port/pool.
Don't use bitminter's flash miner if you want to mine alt coins with your gpus while going after bitcoin with the ASIC cards (it doesn't play nice if you have wallets running or cgminer working and you'll get a lot of rejects)- but by using a command line miner for sha-256, I can also run guiminer-scrypt or cgminer to hash out alt coins.
So is it all worth it? Well.... No. With the 4 erupters running 24/7, @2.5W/ea I use about $0.02/USD of electricity while generating on average $0.12-$0.16 in bitcoin per day. In addition to the BTC, I generated about $0.03 in NMC. Altogether, a daily profit of about $0.15 per day USD.
Here's the breakdown as of today:
4xUSB erupters @$14.87/ea + $42.94 (USB Hub & Fan) + S&H = $106.16
Electricity (@$0.0616/kWH) $0.03/day x 98 days = $2.94
Total Expenditures: $109.10
BTC Generated: .03048521
NMC Generated: .09160374 (currently 0.00053247 BTC)
Based off of the current NMC exchange rate, my erupters have generated a total of 0.03101768-BTC in under 100 days.
At the current BTC/USD exchange rate of $578.86, I've been able to generate $17.96 USD. So after nearly 100 days of mining with the block erupters,
I'm sitting at <-$91.14>USD.
Good luck to those who still try the USB-ASIC route, I'll be mining DOGE and MINT with the cool kids :D