I like to think I am experienced (been programming Java for about 5 years total although I have a lot to learn), and I have taken both the OCJA and the OCJP. I have both the OCJA and OCJP on Java 7 and I passed the OCJA 8 exam when it was in beta.
Being an 'experienced' Java programmer, I spent only a few hours studying for the OCJA, because I know and use most of the stuff on a daily basis. So there is a lot less time investment into studying than a person who has never seen Java before. It's also interesting to see what I get on the exam because it shows me what I don't know about programming, both during the exam ("I wonder if you can do that"), and after ("I got what wrong?"). Granted, a lot of the questions you get on a certification exam are different from everyday life (because you are totally going to put the main method in an enum on your company's flagship software), but they're great for showing you what you don't know about Java, and conversely, what you do.
Getting the piece of paper is also nice, although it means less because it's not something I had to work towards. If I were to switch jobs, that piece of paper may or may not be useful to me depending on who sees the credential. If I stay at the same job, my employer recognizes that I wrote the exam as continuous improvement, which gets me brownie points (and hopefully more $$).