Good question.
I did everything 'by scratch'. It is a great way of learning, its exprience, plus you have complete control over your own 'development system'. You're own headers for apps specifically for you (LCD's, temp sensors, motor control, anything! On the flip side, whether you see this as a good thing or bad thing, you may get stung by various quirks of micro's. For example, noise on the power line causing random reseting, layout issues with a crystal oscillator etc.
After spending a few years with micro's now, I wish to god I had bought a ready made kit
I love my own dev boards, I use them every day, and they are completely customised for my purposes. But.....they took a long time to build, design (perfectionist right here) and meant I spent a large portion of my time on 'hardware' as opposed to actually learning how to probgram micro's.
So...ultimately, if you just fancy a nice project if PCB design to add to your experience that will allow you to use micro's for a specific purpose then sure, DIY ftw.
But, if you, like the vast majority of people here, wish to learn about microcontrollers, programming them, finding out just what they can do, and using them in future projects then by all means
buy a dev kit It will get you programming in no time, without having to worry about the power supply, oscillator or anything else as it should be all there. Also remember, many companies design and sell these things....they must be doing something right.
Then, should you wish to make your own later, or use a micro in a specific project you would have built up your knowledge of software already, and for hardware, you can just clone idea's from your dev board
(I'm sure many of us know the wonderfully simple '7805 regulator' circuit of by heart).
Just my opinion, I started with micro's the hard way and probably wasted far too much time on hardware rather than learning good software techniques, which IMO takes a lot longer.
Blueteeth