Xrumer is fun to play with, that's for sure. I'd rate it's effectiveness at 85%, not as low as 70%. Their programmers are pretty damn good too, not to mention they are releasing a new update shortly to increase that effectiveness rate to at least 95%.
Xrumer is a link/forum/blog/guestbook spammer. As a forum owner myself, I hate the spammers, and we've got some pretty damn good defenses up against them. It's very rare that one actually gets by, but they usually fall into moderation land before their posts are able to see the light of day.
If I were the Xrumer owners, I'd charge at least $3k for it, so to scare off the newbies and people who will misuse it and bring down the accuracy significantly. A buddy of mine uses it, and he continously ups his rankings from his spam pages, and his conversions and traffic are pretty nice. I don't think he's ever made less than $2k a day with it. But like everything else, it takes hard work, which can be repetitive and frustrating, but the end result can yield you some nice cash.