I write my own. I find it easier to customize my own scripts because I wrote them. I find anytime i download and install someone elses script, i end up re-writing most of it because it's either very poorly written or so messy, i can't work with it
Easier to change things when you know how things work from writing it. It also makes it more memorable if its a custom script/design. Just think of the hundreds of myspace clones - you probably wont remember one of the sites built on the same scripts, but one that is totally unique has a much better chance of being remembered.