Granted, you can save a lot of money building your own system.
You get more bang for your buck in the performance arena, and you don't get any second-rate hardware the company installs to save money.
The downsides are significant, however. When you build your system, YOU are tech support. If something starts to go wrong, its up to you to figure out what is wrong, and then call up the appropriate manufacturer about warranty info. Many average users don't have enough experience to handle this, and having Dell (or whoever) to walk them through troubleshooting can be a big help.
The other downside is compatibility. Often there are subtle incompatibilities between various components. Buying from an OEM avoids this frustrating problem, but it basically comes back down to being your own tech support.
Lastly, you really can't build your own laptops yet. Just not feasible. It will be great when you can though, as it will open up a new world of flexibility.
As to the original poster's question... is this a laptop, or a desktop?