If you've ever gamboled online a fair amount like myself(poker mostly) you get these in the mail ALOT. Usually the catch is you get to keep the money you won on their dime only if you deposit money into their site after the trial period and then play more games after you deposit risking both the money you won and the money they gave you, before you are allowed to withdrawl.
Further more usually during the free money periods they put excessively small limits on the games so that you can't just take the 500$ play a few hands of BJ and try to turn it into 2k or something. Often they'll limit it to 20$ hands. More hands at a lower limit means less variance. Good for them bad for you, as fewer people are likely to win big and the losers wouldn't deposit anyways,
The software on the CD is probably legit though feel free to scan it or whatnot, and as for 8 people, well i think their printer malfunctioned and it meant to say 8,000,000 people.
It can definately be fun to do, and you can in theory make money off these type of offers. Often if you play a low variance game after taking advantage of these bonuses(read tons of hands at very low stakes) you can come away with a profit.(the bonuses have a positive expectation for the player so you may win or lose but over the long run youd have more money then you started with.
some wiki info.
http://en.wikipedia.org/wiki/Online_casino#Bonuses
Reminds me of one night i decided to take advantage of a sticky bonus at a site, during a night of drinking. Deposited 200$ got a big bonus and ran it up to almost 10K(the positive side of variance) before the alcohol took control and we really went donk wild. Only time i've lost 2k on a hand of BJ. After loosing most of the money back(the ugly side of variance) I ended up cashing out barely more then i put in. But it sure was fun and makes for good reminiscing when we do get drunk still.
If you do decide to give it a try I would strongly suggest learning solid strategy.
www.wizardofodds.com
and sticking to reputible casinos, you can find a list of problem ones here
http://www.casinomeister.com/