Fellow member Oscar was kind enough to send me some samples of these, 2 each in 4 1/2 X 1/4 and 7 X 1/4 with the 5/8-11 threaded arbor.
Now that I had a chance to give them a real workout on both CorTen (similar to A-36) and 304SS, I have to say I'm very impressed. We purchase the Pferd brand at work, so I got to compare them side-by-side on the same workpieces. I have to say, the cut speed of the 3M disks is in the neighborhood to 3-4 times faster, with less effort.
There is an illusion that they don't last very long, but when you compare inches ground, they are comparable. The time savings alone causes me to feel they are worth the money. (They are a bit pricey, especially for the Harbor Freight crowd... I didn't look at bulk prices, but individual 4 1/2's were around $12, and 7's were around $18 on Zoro.com)
I'm quite pleased with them, and will ask our purchasing agent to put them on his radar. My time is considered very valuable to my employer, and anything I can do to measurably improve my own efficiency is usually approved.
If you're making your living based on how fast you can finish and move on to the next, these are a great time saver for the most time-consuming part of any job, the prep work.
A big thanks to Oscar for his generous gift! I did the math... and there's the shipping...