I agree that it is worth it to buy a new one if you plan on keeping the car for a long time. Sometimes you hit the lottery and get a good rebuild,but most of the time they only last for a short time. I had a shop rebuild my last alternator that I brought in,and it lasted six months. They were recommended to me by someone and needless to say I won't be back. They also broke off a bolt that holds the brush holder regulator assembly in and replaced it with a sheet metal screw that fell out.
I kept the core, and will buy the pieces to rebuild the unit myself.
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T