I've got a fairly tough beard, and pretty sensitive skin. Was tossing the Mach 3 after about 5 shaves, on average. I probably could've gone 6 weeks on a blade, but by the start of Week 3 the results would've been seriously not pretty.
Currently, I'm shaving with a Dovo Bismarck straight razor.
Although I've heard plenty of complaints about the cost of the Mach 3, I never really worried about how many shaves I could get out of a razor blade. I mean, the local store would occasionally price the Mach 3 cartridges at 16 for $22.95. So that's... about $1.43 or so, each. Assuming I shave once per day, and get a mere 5 shaves out of each blade, that's around 28 cents/day.
Call me Bill Gates, but that's just not big enough money to matter all that much to me. Especially when the Mach 3 honestly did give me a good shave.