If you want to trade at Starbucks, then the speed is more than sufficient. I tried it (didn't trade though) and it was faster than my home DSL.
However, don't plan on leaving your seat inside. The range is very limited.
The cost is $30 per month. AT&T has "high bandwidth" unlimited wireless data access for $70. If you need the speed and want to go wireless, this is your best bet. If dial-up is good enough (and it is if you're not daytrading stocks), try a $30 per month data subscription from T-mobile.
Until more wi-fi hotspots become available, they're going to be good for little more than internet surfing.