I took my Speed Pro for a 12km ride through the busy streets of Singapore, in the remaining hours of 2008. Initial experience was very positive. The Big Apple up front allowed for climbing and going off kerbs (short cuts) with full confidence, and as expected, it provided for a very plush ride. It helped too that the SP had the Pantour suspension hub. Speed wise, I felt that it was not compromised and somehow felt livier than the Marathon Racer set up I had. I maxed 45.9kmh on a slight downhill. Perhaps all the hype about the Marathon Supreme had some merit after all.
Wider Front, Narrower Rear - A wider front tire makes sense in many applications, however, when handling and ride comfort are considered. A wider tire will generally provide better cornering traction than a narrower one, assuming appropriate inflation pressure.
A wider tire also provides superior shock absorbency. I personally prefer a slightly wider tire in front, since I suffer from some wrist discomfort on occasion.
Off-Road Issues - Bikes that are used some of the time on loose surfaces often benefit from a wider front tire, with a fairly agressive tread, coupled with a somewhat narrower, smoother rear tire. The wide, knobby front tire will provide the all-important front wheel traction. If your front tire skids, it almost always leads to a crash. For riding in soft conditions, such as sand or mud, a wide front tire is essential. If the front tire sinks in and gets bogged down, you're stuck. If the front tire rolls through a soft patch OK, you can generally power the rear through to follow it.
The narrower, smoother rear tire will have lower rolling resistance. Since most of the weight is carried by the rear tire, rolling resistance is more important on the rear than the front. If the rear tire slips, in most cases the worst that will happen is that you'll have to get off and walk.
This is a great idea that developed out of BMX racing. http://www.sheldonbrown.com/tires.html