I love them, having worn specialised and shimano shoes in the past. The Sidi’s are certainly the best I’ve had. The do come up small, and narrow. My advice would be to find a stockist, try them on to find your correct size and then look around for deals. I got mine a few years ago through CRC much cheaper than anywhere else.
I wear them for my 24mile commute, weekend rides on and off road, and some CX in the winter. They still come up like new after a good scrub.
The footbeds are thin, but I wore mine last year on a London – Paris road ride doing in excess of 85miles a day and didn’t find them excessively painful to wear.
On the slipperiness factor, I don’t find them too bad at all. Maybe the mud up here is more grippy.