Would it be worth it to use a single usb 1.6gh/s antminer? I'm not trying to quit my day job, just have fun with bitcoin.
If it's only for fun and you are ready to make a loss then yes.
If you want to make a profit then you would need to get it for a low price. Say around USD 10 per U1.
But don't expect to see much from this investment.
Here is a calculation:
http://mining.thegenesisblock.com/a/8ab1fe366c