One thing that Sidi shoes are famous for is their gorgeous slipper-like comfort, from the very first turn of the pedal right through to the last one. However, we'd struggle to apply that to the Diablo winter boot. They're not so uncomfortable that you can't wear them, but they're not exactly luxurious, either.
They're also not the warmest shoes to wear in very cold weather, either. The slim, racy looks come at the expense of some of the thicker insulation that Northwave and Shimano cram into their uppers.
However, the Sidi Diablo is the most waterproof boot of the three. We rode the boots for a month through trough-like trails, and always had bone-dry socks at the end -- which is important when you're going on all-day epics in this weather.
Like the Northwave, the top cuff isn't that watertight, but it's better than the Northwave's so is slightly better at keeping water out.
One thing worth mentioning, especially if you intend to ride trails that you're expecting to be littered with roots and rocks, is that the Diablo forgoes Sidi's SRS sole (which uses grippy rubber tread lugs) in favour of the old plastic type. These grip okay over grass and mud, but are very slippery on wet roots and rocks.