Answers to this question will vary!!!
I have never had a problem with Kies. I believe a lot of people's issues with Kies (and this is just a swing in the dark) have to do with their computers as well. I have a custom built rig made to do some serious work. I have never had Kies lock up on me. I have backed up numerous phones with Kies before factory defaulting or backing up data before flashing a custom ROM from stock.
On the other hand, I have heard many nightmares about Kies. If I had to guess, I think it the Kies software is JAVA based (I could be totally wrong) and requires some umph to run it the way it was meant to be run.
All said and done, most people hate it. I have found very few people that actually like it. It works fine for me, YMMV.