MT91 does actually look very robust, do you find them OK for looooong rides? They look tough but they look like they're heavy.
I don't think I've done much more than about 100km in them. They are indeed heavy, though in winter that's the least of my problems, so I'll defer to a proper cyclist for verdict.
They're probably the best cycling footwear for walking in, which can be useful in itself, but probably isn't what you're after.
I love my MT91s. I live in them when touring, I don't carry any other footwear, all weathers/time of the year. I also commute in them 6 months of the year in the UK.
As Kim says, they are proper walking boots, vibram soles, laces, etc, plus cleats. I tend to do a fair amount of walking — unmade mountain roads on fixed wheel, it's a given — but also without the bike. But I also cycle up to 300 km /day in them. With the right socks (base + outer) for the weather they are totally versatile, freezing to 40 degrees.
Cons: the soles are not very stiff, they can roll outwards. The tops are open, in persistent rain it will run down your legs into the boots, and being waterproof they will fill up. Heavy compared to most cycling footwear.
Pros: Comfort. On long rides comfort can make or break a ride, I've suffered all the usual cycling aches and pains but since using these, never with my feet.