You might struggle to get many replies on adding syrup on this site... most of us are having a great time with fresh coffee and the amazingly different flavours from different farms and processing methods.
However, I judged a coffee comp a couple of months ago that had a percentage of syrup coffees in it. DaVinci Gourmet were the supplier of the syrup and I found a couple that I would have again.
I have no need to ever put syrup in a coffee but I can see someone wanting to do it as a different drink or as you said, a desert drink.
Roasted Almond was easily the best one I tried on the day, I cringed as I bought the cup to my lips expecting "plastic, chemical flavours" but instead was happily shocked that this was indeed a good drink (not coffee, but a darn good and interesting drink).
Different Baristas had a different methods of adding syrup to the comp drinks and those that added it before the milk tasted much better than those that added it after. Its too easy to layer it afterwards making an uneven taste through the cup.
As for how much... I suggest you just give it a try and do it to suit your own taste.