A frequently-asked and contentious question...have a search through the forums and see some of the past discussions. Just about anyone who has purchased AppleCare and had a major repair covered by it thinks it's the greatest thing since sliced bread. Just about anyone who hasn't had a major problem thinks it's a waste of money.
Think of it as insurance. On average, Apple makes money on AppleCare, so in that respect it's not a good deal. But what you do get is an exchange of the certainty of a known dollar amount for the uncertainty of a possibly-large repair bill. If you can afford to self-insure (ie. pay for repairs out of your own pocket), it's generally smarter to do that.
Also investigate the terms of your credit card if you plan on using one to purchase your new Mac. Many of them automatically double manufacturers' warranties free of charge, so you'd get a standard two-year warranty (one courtesy of Apple and one courtesy of your credit card) if you can pull that off. That makes AppleCare look even less attractive.
Even those who are proponents of AppleCare frequently suggest that it's a better deal on laptops, which are difficult to repair yourself and expensive to pay others to do. Towers, like the Mac Pro, are easy to open up and swap parts in and out for many things. So if you can absorb the labor "cost" with your own time and know-how, that also makes AppleCare less appealing.