Whether or not you'll like Guild Wars really depends on your style of play. If you're more a lone wolf type of player who isn't keen on joining Guilds or picking up groups of random people, I wouldn't recommend it. The solo play in this game is non existent, certain builds can solo certain areas only. Later in the game the difficulty ramps up significantly meaning a good group is a must, or you'll find yourself repeating the same missions over and over and just not advancing.
If you're a community oriented player that likes the idea of a Guild or meeting and playing with lots of different people, you'd probably enjoy the game. $6 will net you a lot of content, and they do a lot of different events to keep things interesting for even seasoned players, especially during holidays.
In the later add ons like Nightfall and Eye of the North they add computer controlled "heroes" that can be customized to use certain skills and wear any type of gear, that gives a little more freedom to people who prefer solo play, but you can only group 2 at a time, so you will still need help from players in certain areas.
For $6 I'd say it's worth a try, you could pretty easily recoup the cost by selling your account to someone, transferring ownership is pretty easy using just e-mail addresses. The amount of content GW has is its main draw, the gameplay itself isn't very exciting to be honest. It's one of those click and wait style games that's more about timing than actual action. You don't get much control over your character, with the ranged classes you pretty much end up just standing face to face with a mob shooting arrows or projectiles until one of you dies. It's not like Diablo type hack n slash games where you can hit and run.
But since it's free to play because it's not technically an MMO, it's something you can come back to months or even years later to get a little more bang out of your buck.