As Tekbow mentions, and has been noted here before: Shave quality improves with improved technique, regardless of razor used. While my razors were getting their first hone, I was too anxious to wait around and began learning lather and applying shaving techniques to my Fusion/Mach 3 use, and shaves did improve significantly.

Also, while I could get a very fine shave with a Fusion, using soap and brush, and good technique, I could still only get about three good shaves with a cartridge before a rash of ingrown hairs and subsequent irritation set in. Then it was time to pop in a new cartridge. Using 8-10 cartridges a month was more expensive than I could stomach, so I tended to suffer with an irritated neck.

My straights on the other hand will shave for a good month without a touch up honing (which costs me nothing after spending $8 on a Swaty barber hone). I get a clean and irritation free neck for that whole time, and have the added satisfaction of not throwing so much money away each month.

So short answer: I get just as good of a shave with a Mach/Fusion as with my straights.
Long Answer: My shaves are financially, ecologically, and aesthetically superior with my straights.

Peace,
Jim