It all depends on your personal preference.
Go into the "Coop Design" section - you'll see LOTS of photos of peoples homemade coops and they are AMAZING!
I built mine, but never thought to put all the creativity that others have put into theirs. But it was a fun project that my fiance and I got to do together and get a sense of accomplishment out of. We probably spent around $500 on it. After finding this site, I can't wait for the wind to blow mine over so that I have to rebuild!!!
Another benefit to building, is that you can customize it to your own preferences and/or needs. And you can always build it in a way so that it will be easy to build additions on later.
Store bought is nice if you have the $$$ for it (some are pretty expensive!) or if you don't have the time or carpentry skills to build it yourself.
Good luck with what ever you decide!