I'll echo the opinion that it's usually better to build up a bike yourself than to buy one complete. It is not less expensive, at least not apparently so, or not at first... but it's more economical in the long run.

I currently own five bikes, and I've been through three others in the past few years. I got most of them as frames and built them up, part by part. One of the few that I got complete has also been modified and upgraded a lot to suit my preferences. I'm convinced this process gives a guy a much better bike. It also gives me more satisfaction and better appreciation for the bike.

I sometimes see killer deals on new bikes, and I think I could have saved myself a lot of money and work by buying one as opposed to building. Then I look closer and realize the factories cut a lot of corners and use cheap parts to keep the prices so low. I've also seen a lot of friends buy some pretty nice bikes and still have to spend money on upgrades or modifications to get them dialed in right, or even on repairs.

One thing I'm really certain of is this: cheap bikes (whether factory built or custom built with low-end parts) are only good for occasional/casual riding. If you're serious about biking, you need to spend the money for good stuff. Doing otherwise is a sure formula for disappointment and frustration.