First of all, don't believe the user reviews on Amazon or other big box stores. Most of them are either people who've ridden the bike a couple of times or people that had a negative experience. They either love or hate the bike. These reviews are of negligible value.
IMHO, you probably will need to spend about $600 on a new bike. $500 for a Nashbar or BikesDirect bike online then about $100 for assembly and pedals (if needed). If you can stretch your budget to $700 to $800, there are some perfectly fine bikes available at retail, you just got to find the right store that isn't trying to sell above your pricepoint. Most recreational cyclists have bikes that are much nicer than required to get the exercise and the rides in that they enjoy.
My one issue about buying used is that if you know nothing about bicycles, then you are vulnerable to being fleeced. I'm not sure about your area, but where I live the pricing asked on craigslist for used bikes is criminal. You can buy much better NEW bikes at the prices that some people are asking for their old bikes, AND you have no guarantee their old bike is any good.
If you are going to ride 10 miles 3 times a week, then maybe you will be okay with a $300 bike. But if you end up riding more then it may not hold up as well.