I guess it really depends on what you want out of riding.
I use mine for road and training and general fitness.
I try to ride moderate rollers avoiding long seep climbs.
We don't really do "flat" here in Switzerland.
I've mounted a front brake to assist on the steeper descents.

I ride a 42X17, and can for short intervals sustain about 36mph on the descents.
I find it a good 17-18mph cruising gear.
This is about as physically small gear set as I would use.
If I was buying a new set I would go with a 48/49X19.