Wow, there is so much for less than $600 that is good. Go to your local and just get a feel for some bikes, and also play around on your friends stuff. Get an idea of what the length and everything you like. If you order anything you really want to know what size you prefer, or else it will be horrible. I went to my local, found a semi-nice bike to start, and then started to change certain parts of it. So far I've spent a total of about $250 on it and I'm the happiest I've ever been with a bike. So really, just get a cheaper bike from your local with good geometry for you and start changing what you don't like on it. I'm sure you could have a really nice bike with parts you completely like on it without spending over $500, whereas if you got a complete for $600 you would realize that the gearing is wrong or the pedals have no grip and have to spend more than you planned on.
Wow, I thought I was making my posts short, but look, I failed.