I bought one of these mesh seat covers from coolcovers a couple of months back. It was great during the recent heat wave. I dont find it slippery at all, but I wear riding jeans not leather or textiles. I could imagine it being slippery with textile trousers.
Re the fit, i found it more difficult than expected to fit and was wrestling with it for 30mins in the end, had it on and off completely 3 times until I was happy with it. It did take a couple of 50 mile rides to bed in properly to the shape of the seat.
Great mod for the cash :)