As to whether AppleCare is worth it or not, that's a matter of opinion. You'll find lots of opinions on both sides, with roughly 75% saying it's worth it. You really need to decide if it's worth it to you.
You can buy AppleCare any time during the first year warranty period, so you don't have to buy it at time of purchase.
You can check your remaining warranty and/or AppleCare coverage here
AppleCare will extend the 1 year warranty for an additional 2 years, for a total of 3 years coverage from the date of your Mac purchase.
It also extends telephone support from 90 days, which is included with your original warranty, to a total of 3 years.
You cannot buy AppleCare again or renew it once it expires.
Neither the Apple Warranty nor AppleCare will cover damage from accidents, spills, etc. They only cover manufacturing defects.
Neither the Apple Warranty nor AppleCare will cover batteries that have worn out. They only cover defective batteries.
If you want more information, you can search the forum, where you'll find dozens, if not hundreds of threads asking "is AppleCare worth it?" The overall consensus seems to be about 75% in favor of it.