I had one for a while. Short version, you come within 50-100 fps of a 300 WM with recoil closer to 30-06 than 300 WM in a more compact rifle. Technically the WSM is a hair more accurate, but the average shooter won't notice the difference.
It should probably be compared to an "improved" short action 30-06 rather than a 300 WM.
The Specifics:
Since the case burns powder more efficiently you need 10-15 gr less powder than 300 WM, yet it comes close to the same speeds. The slightly lower speeds combined with less powder results in much less recoil.
In my 7.5 lb rifles, with my loads
308/180 gr bullet/ 44.5 gr powder, 2620 fps, 18 ft lbs recoil
30-06/180 " /57 gr powder, 2800 fps, 22.6 ft lbs recoil
300 WSM/180 /64 gr powder, 2950 fps, 26 ft lbs recoil
300WM/180 /75-80 gr powder, 3000-3050 fps, 30-33 ft lbs recoil.
I simply never noticed the difference in recoil between 30-06 and 300 WSM. I did notice the difference between 30-06 and 300 WM. It wasn't worth the extra speed to get kicked that hard for me to keep a 300 WM.
Why I sold it. I qualify as an old fart. I've been using 30-06 for over 40 years and I ain't selling them. I've found that a 308 does everything I need a rifle to do. My 308 load and rifles will kill anything including elk past 400 yards and that is as far as I can shoot. Someone offered to buy mine for more than I paid.
But if I were a new shooter with no emotional attachment to 30-06 I think the 300 WSM is the better round. But I also think 30-06 is bigger than 95% of us need. Today I'm inclined to lean toward 308, 7-08, 260 or 6.5 Creedmoor. And I consider those rounds superior to either 300 magnum.