Are air currents at high altitudes predictable? Are they used to save time/fuel on long flights?

they are usually called ‘jetstreams’ and answers yes to both of your questions.

have a look at this article on wikipedia.

http://en.wikipedia.org/wiki/Jet_stream

Yes and yes but they flow west to east only. So good tail winds going that way and you have to avoid them going east-west.

Yes and yes. Winds aloft can be predicted to some extent, and they are also measured directly. They tend to follow certain general patterns, although the exact winds at any particular point and altitude are difficult to predict for specific times.

At high altitudes, jet streams—high-speed air currents—have a significant effect on air travel. The jet streams generally blow from the west, so aircraft traveling east try to put themselves in the jet streams, giving them a huge tailwind that can substantially reduce the time required to get to their destinations. Westbound traffic tries to avoid the jet streams, so that a massive headwind does not dramatically slow their progress to their destinations.

Air traffic routes across the North Atlantic are changed daily in order to take into account changes in a key jet stream that regularly flows in that region of the world. The routes attempt to optimize travel with respect to the wind for both eastbound and westbound flights.

Yes to your 2 questions.
They are called “JetStreams”
Depending on the season they can be located North the country or South, using them can save you a lot of time flying from US to Europe.