^^This.

While most people here will disagree, a bike is a bike. You pedal, it moves. The main difference is how well it does this. Entry level bikes work just fine. Over time, what you will discover is they may require just a bit more time to keep them tuned. Higher end components tend to work a bit smoother/faster and of course are lighter which is the holy grail of cycling (light bikes). Unless you are looking for a religious experience, buy the bike you enjoy riding and that fits your budget. If you get hooked, mortgage the house, the wife, the kids and feed your addiction.

EDIT: I will say it can be worth it to expand your budget in the $1000 range and look for something on sale to get the most for your money. The entry level model in most lines has been trimmed of every possible upgrade to get the price as low as possible. Look for something with sora level components or better yet tiagra (or apex in the sram line).