I also use my iPhone without a case and I like the way the new matte back looks, especially in space grey. Having said that the XS Max feels somehow better in the hand, mainly because of its glossy back. I like the color of the new space grey iPhone better than the black color of the XS Max though, so all in all the new Pro phones look better than the previous phones..
Another benefit of having a matte finish is that you don't see fingerprints. I cannot say that the new iPhone is more slippery than the previous generation.