A constant speed prop is more costly, more complex, more weight, and more maintenance than a fixed pitch prop. So why do people use them and love them?
Simply put - it's like having an infinitely variable transmission to hook horsepower up into the air. With a fixed pitch prop, it's like having a car with only one gear. You can choose that gear to be very good at one thing and one thing only - your friend obviously has chosen his to be good at cruising flight, which is very common. Others choose it for lower flight speeds (like banner towing or glider launches) or max climb, or a variety of other points.
One side effect of this is that the prop operates part of its time outside the ideal airspeed range. When the airspeed is too slow (at takeoff, for example), the prop has too much load for the engine and the engine can't make full rated RPM - which means you don't get full rated horsepower. At the very highest speed range, the prop doesn't have enough load to utilize all the engine horsepower and the engine will go overspeed, and you'll have to pull the throttle back to prevent that. Either way, you're not getting everything the engine has to offer converted to thrust.
With a constant speed prop, it will change the pitch of the prop as the airspeed and horsepower changes - it is essentially "changing gears" to allow you to set the engine at the best RPM to make the best power. I can turn the engine at 2700 rpm for full power at takeoff, and dial it back to 2400 for efficient cruise at high altitude, leaving the throttle wide open the whole time - something that no fixed pitch prop can do. It gives you large advantages in takeoff power, climb rate, descent rate (acts like an airbrake when you pull the power way back) and still gives very good cruise performance. It's a combination of operational advantages that makes it worth the cost, weight, complexity, and maintenance. Kind of like a girlfriend or wife.
The larger the range between stall and max cruise speed, the better a constant speed prop will perform on an airplane. In a Cessna 152 that range is very small and you won't ever see one with a constant speed - the advantage does not outweigh the cost. On RV's I suppose you could say it's a toss-up and people go either way. On faster airplanes constant speed is the standard.