Ok I've heard a lot of people comment/complain about dating sites, and me and a friend actually made profiles just to see what it was all about. I was not looking to date.
Anyway, I had the profile up for 2 days. In those 2 days, I got about 60 messages. 2 of them were from girls, despite my profile saying I was interested only in men. 1 was from literally a 60 year old man. One guy was apparently a male model and set me pictures of his ripped self in underwear, asking me if I wanted to hook up.
The rest were from men who clearly did NOT read my profile. I had smokers messaging me despite my "No smokers" thing being checked. I had men with kids message me despite me not only saying I didn't want to date men with kids, but also me complaining IN THE PROFILE that I do not want kids and I find them to be annoying. Most of these messages were in text speak and said something long the lines of "hi wow ur really hott . msg me if u want 2 get 2 kno each othr". And "hott" was one of the nicer, more CF appropriate comments. I got much worse.
Out of all that, I had just one legit reply from somebody 5 hours away. He is the only one I replied to and ended up talking on the phone with. We actually did meet and hang out and though he's not somebody I am interested in dating (he knew my intentions were not to find a relationship from the start), I did make a good friend.
So I guess the moral of the story is that there are good people on those sites. But you are going to have to go through a lot of crap to find them. At least in the case of women. You will get hit on if you are the least bit attractive (even by other women), you will get inappropriate remarks and possibly inappropriate pictures. And probably worst of all, very few people will actually read your profile, what you bothered to write out about yourself, no matter how detailed it is. And it will be obvious.
In the case of guys, my couple guy friends who've made profiles would send PM's, but rarely ever got a reply. So I can't say what would happen for you.