Download the Speed Test app from the marketplace and test your 3G speed. Whatever you get on your phone is very close to what you'll get via the hotspot.
3G speeds average between 600kbps and 1500kbps, with it topping out around 2000kbps for download speed. Pings are usually between 150ms and 300ms, but can spike quite often. Don't expect to be gaming well over it.
If you need a RELIABLE connection, this is not a replacement.