it takes me roughly about 4-6 minutes more useing my DE then it does my mach3.
I do get a better shave and NO irritation. i think that going slow and keeping my face wet has really helped in that. when i used the mach 3 (and the foam shaving cream) i would always have irritation.
Now i take my time, i have blades i like, cream i like and take my time. sure its slightly longer but well worth it (to me).
i do see how it's not for everyone. if you don't get irritation then teh mach 3 or such is fine (had worse irration with razors after mach 3 for some reason) it's faster and only slightly more expensive.
Sums it up for me. I get a better shave for a couple extra minutes of time and significantly less irritation.