Mine has taken terrible abuse. Stuff my SLR's and even my standard M's could not take. The water would be a problem but what mine has taken has convinced me that is is extremely rugged and the case can take abuse that would make some and me cringe. I didn't buy it to show it off as still new and it doesn't look it. I bought it to use when and how needed.
I'm intrigued. What couldn't your SLR or 'standard' M take. What broke? What type SLR was it? Which M let you down and why? Did it cost $7000? Was it a pro body SLR?
I'm not having a dig at you personally but a post like this needs some backing up because it doesn't make sense without example.
To the original poster, why ask in the first place? Are you planning a major expedition or do you just want to make sure it doesn't pack up while shooting the kid's birthday parties?
Seriously though, does it matter? I would personally rate any Nikon F, F2, F3, F4, F5, D700, D3 etc, Leica M6, MP above any M8, M9.
Horses for courses. The M8 and M9 are not pro cameras. They are digital M cameras riding on the back of mechanical M camera longevity.
What is toughness? Is it a measure of reliability or is it the ability to take knock after knock, all weather, sand, dust etc and continue to work?
I've had an M9 and all the cameras above and if I was allowed to take one camera and only one to get unrepeatable shots anywhere on the planet it would be any one from the Nikon list or an M6/MP. OK I didn't shoot the M9 a lot but it never gave me the at ease feeling I get from an M6.
I'd bet that a very high percentage of M9 users always have a backup with them because they don't trust them.
If you can afford to get an M9 and not get too annoyed when and if it packs up then go for it. There are plenty that have been reliable under duress but equally there have been plenty of break downs. Some are lucky, others are not.