Joined up to one, well more of a chat site in December of last year - with no intentions of ever meeting anyone from there in person and was just looking for a chat (the idea of meeting someone from the internet was just weird/odd/slightly terrifying to me) but I was upfront about that.
BUT then in January I signed up to a "dating site" again wasn't actually intending on meeting anyone (was upfront about it).. I just wanted to see what kind of guys were out there. On the second day of being on there I stumbled across this guy - I inniated contact (which I hadn't done previously with anyone else), talked every day for about 10 days, met up and have been together for over 6 months now and things seem to be going pretty good.
I think online dating can be very hit and miss and its hard to judge someone's character through text - but if you put in the effort and I guess nurture the conversation, and persevere you could find someone worthwhile.