This is just how I feel about sugar in bad espresso...
I think it makes it worse. Without it, it is thin, bitter, and hot. Adding enough sugar to get rid of the bitterness is near impossible, because the drink becomes this sickly sweet, yet still bitter sludge. For me, anyway. YMMV.
On a different note...
In light of reading through this thread this morning, I decided I would add sugar to my morning brew(today was PressPot Panama, La Berlina Estate). To a 12oz cup, I added half a tablespoon of raw sugar and stirred. Did I think it was bad? No. It wasn't bad, but I much prefered it without sugar. The sweetness wasn't a nice addition to the cup, though it was FAR from undrinkable.
I wonder if there is a correlation between adding sugar and adding cream to coffee. I very seldom see someone add sugar without also adding some sort of dairy. Then again, this is based on what I see in gas stations and 7/11 stores, so.. take it with a grain of sugar
If you enjoy adding sugar to your coffee, espresso, cappa etc etc... I don't think there is anything wrong with that at all. Make YOUR coffee how YOU like it. What gets to me, is when it becomes habit to add sugar, and someone just isn't open to trying a coffee without it.(not referring to anyone here, friends of mine do this.)