Internet rates vary by cruise line but most cruise lines charge around .75 per minute or a 250 minute package will run you $100 or .40/minute.
Speeds can vary from unusable to reasonably fast. Most of the time it will remind you of old time dial-up. Don't expect to upload or download large files.
It can be good for checking email or some quick browsing.
Make sure you suspend your updates. On my last cruise a person was sitting by me in the lounge and both of us were using our laptops. He was cursing that it was ridiculously slow while I was having reasonable response time. I took a look at his laptop and the Systray and saw that his Window Update was running as well as his virus software update. Ouch! That would slow down a DSL connection. I terminated the updates and he didn't have any speed issues.