I don't custom design classes for my campaigns, but I have written classes. About 3 years ago, I wrote a Bard class for Knockspell Magazine (last issue of I recall - I'd hate to think my bard killed a magazine) and I wrote some Halfling racial classes for S&W a few years back. I neither case were the classes written for a specific campaign.
That being said, I do know that some DMs do design specific classes for their campaigns or tweak existing classes to make a better fit.
Do you write such classes? Have you played in a campaign with custom classes? Pros and cons? Tell us a bit about your custom classes.