When I'm manic I've used these sites and found most people are ok. Though I have met a few guys who want more than a one night stand. (I only want sex and nothing on-going when I'm manic) and keep contacting me for weeks after, but I've always stressed that it a once only event and a quick **** when I've met them.
It depends on what you want. But always play safe and negotiate the terms of the encounter before you meet.