First, is the $500 for bike only or bike and whatever stuff you need to get going? That will make a difference. A friend bought the Dawes Lightening Sport ($260) from BD when she started college last fall and it has served her well. The reviews on it seem pretty solid for a low end bike. As you move up in price, the components get better and will most likely last a bit longer.

If there is a Performance Bike Store near you, I'd suggest checking there. If you catch a bike on sale, it can be priced close to a comparable BD bike. Plus you can test ride it as well as having a place to take it if you have issues. I bought my Schwinn Le Tour GS there for $500 on sale (list was $750). It is a Sora/Tiagra mix similar to the Mercer Orion Al on BD. I joined the buyer's club for $25 got $50 in extra merchandise (ended up buying a couple of nicer road bikes later in the year so the club really paid off).