I would wait until your original warranty is almost up. Then it wont seem as bad. For a powerbook it is worth it. They cost a lot more than 250 to fix just about anything on it. If one thing breaks and it would have cost you 400 then you saved 150. Apple makes great products but stuff happens and they are not perfect.
I purchased a powerbook in september 2003 and purchased the apple care plan. Since then I've had to send it twice for a lcd replacement. A new keyboard, and a new connector for the power cord. (this wasn't a refurb model). I'm glad to say I had the apple care plan cause god knows what it would have cost if I didn't have the plan
For under $300 you have piece of mind. I would get it without even thinking about it. I do not have it, but my PB is about 3 weeks old. I am still under my 90 days of tech support w/phone and then well within my 1 year. I intend to buy apple care in August, that is because many of my families birthdays are over and I can drop the $300 without feeling like I am taking away from their gifts.
I have a 12" powerbook that I'm using RIGHT NOW and I bought the extended warranty for it a day before it was up (being a poor student n all) - except that it didn't ship for ages... So I had to call up Apple and this great Irish chap sorted it all out for me! Anyway, the moral of the story is, get AppleCare but get it slightly sooner than one day before the warranty expires!
Applecare on a Laptop just makes sense to me. I went ahead and factored the cost of Applecare into the cost of my Powerbook when I was budgeting for the purchase. I agree that if you plan on having the Powerbook for more than a year, Applecare has a great potential to save you serious money in the future.