This is more then bicycles. The conventional bicycle despite advances
in construction and materials has not really improved much in the last
100 years. This is mostly because of the UCI who sets the defintion
of a bicycle for most races. A HPV is what happens when we stop
following this forced definition. It also expands from land
into air and waterWhy change our notion of what is a bicycle? The traditional design of a "diamond" frame has not changed much in the last 100 years. As a result the UCI record for a 1 hour ride has gone from 49.4 km (1970) to 49.7 kmi (2005). This is the result of Al and carbon frames. Once we allow the geometery of the vehicle to change, the hour record is greatly improved. As of 2002 the HPVA record for the hour is 82.6 km. Lance Armstrong has suggested that he might try and break the hour record, but I doubt even a great cyclist like him can make up the more than 30km penalty caused by staying within a UCI definition of a "bicycle".
What does that mean to me? I don't race. As a normal bicyclist, the first thing you will notice about an HPV is the improved comfort. It has been suggested that a typical bicycle saddle is unhealthy, it certainly can be uncomfortable. An HPV usually has a seat like you would find on a sports car or lawn chair, this allows you to spend more time on your bike without pain or discomfort.
The second, is that you will go further with the same effort. As suggested by the hour record times, HPV can be more efficient. Faster? Yes you can go much faster on recumbent. Above about 30km an hour most of your effort goes to overcoming wind resistance. By allowing the vehicle to become more streamlined it will take less effort to reach the same speed.