I like leather seats in the Omega, but leather needs cleaning to keep it at its best. Some say black leather doesn't show the dirt like the lighter shades, but if it isn't cleaned regularly, it becomes excessively shiny due to dirt combined with body movement, which causes it to have a more 'slippery' feel. How many cars with leather seats do you see with a shiny drivers seat, but a more matt finish to the passenger and even more 'as new ' matt finish on the rear seats? Leather seats are naturally more 'slippery' than cloth, but regular use of a proprietary leather leather will help to restore the 'as new' matt finish and reduce the slippery feel.
The shiny look is more noticeable on the nappa leather finish, rather than the grained leather fitted in 2003 on cars, but both benefit from regular cleaning.
I regularly clean my leather seats in all my Omegas and my Audi, all of which have leather and the problem of 'slippery' seats is greatly reduced.
Hope that helps....