Are these fun since you can't solo that much with them?

Different from person to person..

So what do they do? Heal ppl in the Cata's?

yes and no

sometimes i find guardian alone is a real chore, although there are many great people about it is all just a free for all of spam and snatching at times and at others he can stall unless you can afford some expensive stat items to lvl up when alone. you will litteraly be stranded.

on the flip side if you can get just one friend, that is all it takes, to lvl with you and build to complement your strengths and negate your weaknesses you will find no better char to play.
get two or three and suddenly things get rly interesting

Ok, so I am thinking Guardin or Mage

my vote would be for a mage, i personally didn't care for guardian

i luv my guard, only thing is when im host of maze cata it sucks cause u gotta control person and attack and heal