I'm a customizer. I customized my first pony a year after I started collecting. Her name is Moonlight and she's a BBE unicorn (the purple one.) All things considered, she turned out pretty nice. I spent months researching customizing before I actually started on her.
Although I've been customizing since 2001, up until 2008 I only had about 40 customs under my belt. Partly this is because I've gone months at a time without working on a custom (getting distracted by other art projects, mainly). Partly, this is because I didn't always have the tools or materials onhand with which to customize, and I don't like to take on a project when I can't finish it.
Since 2008, however, I really started focusing on customizing as my main form of artistic expression. I've found supplies that work well for me: Reaper Master Series paints, Dollyhair nylon hair, and Apoxie Clay, to name a few. Having the right tools really makes a difference in the finished product, as I'll come back to next time.
In the spring of 2010 I celebrated the completion of my 100th custom with a raffle on the MLPArena. The winners each received a custom of their own design. If things go on at this rate it won't take nearly as long to reach 200 customs.