It’s a nice service, but there are both good and bad aspects to it. The obvious good aspect is the massive number of customers viewing Steam, looking for new games to try every day.
However, there’s no telling whether the $100 that it costs for you to be granted access (indefinitely, if I’m right) to Greenlight will be worth it, as it’s not guaranteed that your game would be accepted. It would still need to be passed by the consumers, as you know. Are you sure that your game is of the quality that it would need to be to push it to GreenLight?
Steam gamers are just that - gamers, first and foremost. If something looks, sounds, or plays badly, they’ll see it, and generally won’t be forgiving. It might be best to be sure that your game has the highest quality that you can manage (or, at least, that you personally feel comfortable with) before submitting it to Steam.