First off, World of Warcraft is a great game, so many things to do, and (in my opinion), its great educationally too. It greatly expanded my vocabulary among other things.
As for the 60-day game card, well I think this is the best way to pay for your play. First off, at £7.99/month (currently amazon prize) it is cheaper than 1 month (£8.99/month) or 3 month subscription (£8.39/month), and so (if you have more than a month to play it of course!) then you are best off buying this. A 6-month subscription is cheaper slightly (£7.69/month), but I would say that the flexibility to stop playing (something could come up with work etc) is better with the 60-day - and helps you see how much you've been playing (i keep a stack of the game card boxes - it soon mounts up!)
Further, 60 days is great for a summer holiday of playing if you are in college!
Finally, the game-card is extremely easy to use. You receive the card (comes very quickly from amazon in my experience) and then simply scratch off to reveal the code. Enter this code on Battle.net and instantly you are credited with the 60 days.