Is it worth it to do the belly pan: absolutely. The manufacturers - except in a few isolated cases - don't spend much time on the underside of the car. You, the buyer, don't see it and therefore are not expected to care how it looks. So it's full of nooks and crannies and aerodynamic nightmare fuel. Some cars have a pan under the engine to improve engine cooling; if you take your car somewhere to get the oil changed, be damned sure they put that thing back. It's about 30/70 they'll just pull it off and toss it.
The typical ecomodder, however, is quite interested in how air flows over every bit of his car, which includes the underside. Belly pans work.
HOWEVER, and this is a big one, you can get nearly belly pan level results with a deeper air dam. The rule of thumb is to get your air dam as low as your lowest hanging underbody part, and to take it out to the edges of your tires. That's faster, easier and cheaper to do while coming close to yielding the same results. Adding side skirts makes the air dam's contribution that much more effective, and like I said, improves the engine's cooling system performance by creating a low pressure zone under the car.
Suggested reading:
http://ecomodder.com/forum/showthrea...ehicle-83.html
Belly pan - EcoModder
http://ecomodder.com/forum/showthrea...how-15026.html
http://ecomodder.com/forum/showthrea...ect-29902.html
http://ecomodder.com/forum/showthrea...ess-23604.html
Anything Aerohead tells you to do aerodynamics-wise, he's probably right. I think the guy is an actual rocket scientist and Knows People in the aero world. Advice from him won't point you wrong. He's converted a Toyota T100 into something that closely resembles a suppository, it's very sleek and a little disturbing.
Reading other stuff from Aerohead and others (this was years ago so if I misremember, my apologies to everyone) what came out of it was that virtually any attention spent on reducing air drag, on anything, pays off. It may never pay itself off, but it always pays.
UltArc has taken a 300-hp Mustang and teased Corolla-worthy numbers out of it. That required some mods but to look at his car, you wouldn't guess they were there. IIRC there's a belly pan under his ride, too.