I have gotten into biking this year, since I live 7 miles from school in a very bike friendly city (Boston). It is wonderful to have the regular exercise and some time to reflect on life each day. I have recently become interested in purchasing a new bike, and here is what I have found in my searches of what is a good bike to get:
Personally, I like to really push myself sometimes on my bike commute; see how fast I can keep my pace. Keeping a stopwatch going, and pausing it at stop lights gives a chance to compare times and be a little competitive with myself. Also, this makes for great exercise! Other times I take it more relaxed, but I still appreciate being in a more forward position on the bike to keep a decent pace.
I am also thinking that it might be fun to go on some several day touring trips, go visit NYC from Boston or something.
Finally, my budget for a bike is around $500, give or take a few hundred.
As Guillaume pointed out below, for the riding that I do, any new bike less than about $1200 is going to be a waste of money. However, there are a remarkable number of bikes that are of sufficient quality which you can find used on Craigslist for about $400-$700.
For my riding style, I have learned that the best bike for me is a lower end “endurance bike” or maybe a “sport race bike”. Different brands call it differently. The idea is that I don’t want a heavy touring bike (I am not going cross country on this thing), but I also need something that is strong enough to handle the weight of a bike rack with all my books (true race bikes are made too light for their frame to have this strength). The lower end race bikes will often have little pegs in the fork of the rear wheel up near the seat post. This means that the frame is sturdy enough for a bike rack.
The final criteria is sufficient quality in the bike components (shifters, gears, etc). Shimano is the industry standard for these, so that is the best bet for component brand. For road bikes, Shimano has 5 levels of quality – Sora, Tiagra, 105, Ultegra, and Dura Ace (from lowest to highest). Get Tiagra or better. There are two big reasons for this. One, the parts will last longer, are better made etc. Two, the shifters on Tiagra and up have a tab you can reach from the drop-down part of the handlebars; Sora shifters don’t have this so you will have to constantly change your hand position whenever you want to shift… not fun or safe. One other brand is also good for components, although they are smaller and thus less prevalent. Avoid Campagnolo because replacement parts are more expensive and not well stocked by most bike shops (it is an Italian brand)
The final issue is what brand of bike to get. There are 5 main brands I am aware of: Giant, Specialized, Trek, Jamis, and Cannondale. I looked at the bikes they offer, and these seem to be the appropriate bikes from each brand:
Giant Defy 1
Specialized Allez (older ones have bike rack mounts)
Cannondale Synnapse (some models of the synapse are for racing, so look for the bike rack pegs.)
I just looked into getting my girlfriend a bike, and my friend from the bike shop strongly suggested the Trek 7.3 FX (the Trek 7.3 FW WSD is specifically made for women, but the normal version is fine for women as well). We just bought it and it seems like a fantastic bike. Basically it is like a road bike, but you can sit up a little more comfortably, it has a cross bar handlebar instead of drop-down, and the tires are a little thicker to make the ride less bumpy. It is also good for long trips like a 40 mile bike to the ocean the weekends; a very good all-around bike.
To learn what fit is best for you, this youtube video describes it very well.