Here's the thing about Guild Wars: there is NO monthly fee. So it makes a great option for someone who doesn't have the time to progress in a more in depth MMO like FFXI or WoW. Other good things about it are you can buy one, all, or anywhere in between of the different campaigns and still enjoy the game. If you do own more than one campaign, after you reach a certain point you can move your character(s) between them at any time. Giving you access to campaign exclusive spells on characters that otherwise wouldn't be able to get them.
In other MMO's you have to pay usually per character, Guild wars gives you the ability to make 4 if you own one campaign and gives you 2 more slots for every campaign after the first you add to your account. Since the level cap is only 20 being able to make more characters is very nice and lets you experience the different classes the game has to offer without breaking your wallet.
Now onto what I consider the bad: Very little community except for PVP. The only time you'll see other players is while you are in Towns and Outposts(and PVP if you play). The game is structured such that when you go into an explorable area you get your own "copy" of the area for you and your party members(if any). This is both a blessing and a curse I suppose, because it means you won't be competing against anyone to claim that elite monster, but it also means if you run into trouble no one will be there to help you.
At times Guild Wars can feel more like single player than MMO because most of the mission can be done with the help of NPC's. Again, this great if you don't have much time to put into the game, or if you don't have a regular play schedule. It's a game you can play at your leisure This game doesn't discourage teamwork, but it does make it possible for you to get things done on your own.
All in all I've found Guild Wars to be the best MMO for someone who only wants to play casually. It's a game that you can play whenever you find time. You don't have to wait to find other people to get things done, and you can experiment with many different classes without spending extra money.
If it's a game you are interested in trying they offer a free trial of the game
http://www.guildwars.com/freetrial/. If you want to buy the game I'd suggest starting with the Nightfall campaign. It's the easiest campaign to get into and you get access to NPC's that you can give orders to, and assign their skills which makes them more helpful than the typical NPC.