I never really found any appreciable difference in speed while using my studs. That being said, once I switched back to the slicks after the winter was over I felt like I was flying. I looked at it as three months of training. For me the biggest difference in winter riding is the handling; riding over the "mashed potato" consistency snow (that has been compacted and mashed by passing cars) can get pretty squirrelly. IMHO the balancing act made me a better rider.

Like jeffpoulin stated, a back-up bike with more aggressive tires would be ideal, but I found my hybrid and 700c Marathon Winters was good for the majority of my rides. For those really bad days I wish that I had one of these: