RafaelLopez: I'm trying to hook up the trial version here, it's taking a while to download. Later on, can I use my character in my friend's copy of the game, perhaps on his actual account? Am I in for a total mess by starting now with the trial and using his copy later?
Also, is it recommended to buy all campaigns?
Unless they changed the way trials work, what you're downloading right now is the full game. There should be no difference between it and your friend's copy of the game. To play the game you'll have to create an account. Once the trial is over (or even before it's over, if you can't wait), you should be able to add a "full version" access key to your account and keep playing with your characters.
You will not be able to transfer or use your characters on your friend's account, but I dont see why you'd want to do that, since sharing the same account means you wouldn't be able to play together.
As for buying all campaigns, if you're serious about playing GW then yes, it's a good idea to own them all. But there's plenty of content in each individual campaign to keep you occupied for a while.