BIS is the same price as the data plans for iPhone and Android. The original and now grandfathered unlimited plan on AT&T is $30 US a month. If you think that's pricey, I'm on BES and pay a 50% premium -- $45 US a month.
It depends on your usage. Do you like to browse the web and maybe a lot of multimedia on your phone? If so then try out a BIS/BES plan. Nobody knows how you use your phone, but yourself.
Like the others have said, it really depends on your preferences! If you feel your needs are, or would feel you would make good use of the data then go for it! It's quite convenient, but if you're happy with what you have now, no point in wasting the money
If you want to enjoy the Blackberry data services you should get it, i thinki that the best idea will be to try it out and then you will know if you need it or not.
A BIS isn't expensive, it's about 10 dollars a month where I live, in Singapore. But I would suggest you wait for BB10 to come out before plunging into the ecosystem full on, unless you absolutely must have a Blackberry phone keyboard to pound on (or a corporate user) it's absolutely not worth getting a new Blackberry phone at the moment.