A .380 would not be my first choice as a carry gun, but I would certainly love to have the option of a small gun, like the LCP, for times when even my 642 may not be discreet enough.
Effectiveness is relative. A .380 is going to be more effective than a charming smile if you can't carry anything bigger. Assuming I was able to shoot it well, I would be ok carrying a .380 loaded with 90gr XTP, which seems to be the top performer and most recommended JHP load I've seen. Personally, I see the value of a .380 being a carry gun for times when you can't carry something bigger.