What are the pros and cons about building a bike from scratch vs. buying it new?
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The pros of buying pre-built are the cons of building your own.
Each approach is born of very different requirements. That is, the person who is actually likely to build their own is very unlikely to buy off the shelf and vice versa. (I assume that 'build from scratch' also covers getting the LBS to do it for you to your specification).
I would caution everybody thinking of building their own that you will get much better 'value for money' if you buy pre-built. A $1000 pre-built will get you much higher quality parts than a $1000 build your own. This disparity seems to reduce the sillier the price becomes at the high end.
You will need access to some specialized and expensive tools (thanks Gary!) and the skills to go with them.
There is, as always, an intermediate stage where you source everything yourself but have the LBS do the trickier jobs like headset pressing, bottom bracket cups and steerer tube cutting/star nut pressing. Just remember that your LBS is a business and needs to make money - so buy the parts from them if you want them to fit them!
Build Your Own
The list by Bryon is great, but I have one more addition - In addition to buying parts, you need to buy or have access to a number of specialized tools and the knowledge to use them.
If you are like me and find the dis-assembly, cleaning and re-assembly of your bicycle a zen-like and cathartic experience, this is a "pro", otherwise it may belong in the "con" column.
Building from scratch is cool because you really get to build a bike that suits your needs perfectly.
Buying a built bike is cool because the price of the whole thing is usually lower than the sum of its parts. Also, you're sure that you won't mess anything up while assembling the bike.
So if budget isn't an issue and you know what you're doing go ahead and build, but the popular idea is often to buy built and modify some parts as you go.
EDIT: I might add that if you build your bike you get to "know" it really well and it will ease the maintenance since you'll already know how each part works. But you can get the same knowledge by dismantling your pre-built bike for an end-of-season/beginning-of-season maintenance.
Building your own anything (bike, shed, computer) is more about the "yeah, why not?" than the "why should I?"
Most people wouldn't begin to think about it for a whole raft of reasons - ability, cost, quality. But if you're the kind of person to even think the question, most of the cons just evaporate after a little thought, preparation and practice.
Why shouldn't I do that? - no good reason at all, apart from the facts that I know that I probably couldn't actually do it, it would be quite pricey and I'm not sure I'd want to ride the result in traffic ...