During the first few days of service, your allowance resets constantly. This is done as a courtesy to allow people to update their software if they need to. After this honeymoon period, the data you use is counted towards your total allowance.
Satellite internet is much slower than terrestrial internet, because the signal travels thousands of miles from space, which causes high latency. Satellite internet is also much more expensive to maintain, which makes it a more expensive service for the consumer.
To cancel, you will need to call 866-347-3292. Since you're still under contract, you will pay a $400 termination fee. See http://legal.hughesnet.com/ServiceTermsAndConditions-current.cfm
To avoid equipment charges, you will also need to return the equipment (modem, power pack, and the radio from the dish). You will get a kit to return the equipment when you cancel the account.