Are jet streams a net benefit in time and fuel savings?
Yes, Jetstreams are a net benefit on longer flights because it is easy to minimize headwinds and maximize tailwinds for both directions. 

Longer flights can detour hundreds of miles in order to take advantage of more favorable winds and still result in a net gain. 

I frequently fly from Toronto to Hong Kong. We always fly a polar route YYZ-HKG as it is shorter, but usually take a HKG-YYZ North Pacific route over Alaska to take advantage of the prevailing winds.

This daily flight is always scheduled at 15 hours 40 mins for Toronto to Hong Kong over the pole but only 14 hours 45 mins returning via a longer route over Alaska. 

The longer flight routing over Alaska is a net benefit in time and fuel due to the prevailing westerly winds over the North Pacific.  This type of flexibility is not available on shorter flights.

YYZ-HKG 6,787nm non-Stop 15:40

HKG-ANC-YYZ 7,050nm non-Stop 14:45