Is it going to be a keeper?
Your off the rack bike, it rides well, does everything other bikes do and was a great deal. So why would you ever want to get a custom bike? Gathering all the parts and putting it all together sounds like such a pain.
Well, here is my short and sweet answer to this:
I have both, custom and off the rack bikes and they all serve me well. But there is something about a custom bike that makes it very special. It's your own creation, a one of a kind master piece. Usually it starts by falling in love with a frame and then picking exactly the color that you always wanted. If you are buying from a smaller manufacturer who does all the work on site you will also have the option of fitting the frame to your body type. (this usually is not a issue for me since I am 5'9" with normal arm and leg length) Now comes the best part of custom builds, you get to choose all the goodies that will complete your cycling heirloom.
This is definitely going to cost you more than an off the rack bike in the beginning. Most shops will tell you that you should buy a complete bike and that you can always upgrade parts or wheels later. The only problem with that is, once you upgrade anything you have sell the left over parts for top dollars on ebay to justify buying a bike with the wrong parts in the first place. This could potentially be just as much work and expensive as building custom from the get go.
The choices are endless and if you are doing it right you will have a bike for a lifetime.
The moral of the story is:
If you can find a complete bike that has all the parts that you can live with in the color that you want in the right size and you are not planning on changing anything, then go ahead and buy off the rack. But if you are planning on upgrading individual parts later, you might want to consider starting from the frame up and go custom all the way.
btw. "Only bicycle geeks like you and me will see the value in custom bikes, so don't bother making your case to the pedestrian non cyclist out there or they might lock you up. :)"
Enjoy the ride,