The 50/50 Ultra plan is only available in select areas where capacity exists.
When you exceed your data allowance you will be throttled to around 150 Kbps ... about 3 times dialup speed.
If your plan includes "SmartBrowse" your speeds will remain at full speed for "low intensity" functions such as checking email (no large attachment) and viewing of "simple" web pages ... no embedded video or that sort of thing.
Your other option is to purchase "Tokens" that will allow normal speeds. Unused token data will remain in your "token bucket" upon your monthly refill.