This is not the seat I would want on a mountain bike.

Think about it-one way you control your bike is with your inner thighs against the horn on your seat. I read somewhere (can't remember, but it was in a similar discussion about bike seats) about someone crashing with a seat like this because they couldn't control the bike after hitting a bump. Apparently they slid right off of it and down they went. This seat looks like it would be like sitting on a barstool.

I have a Serfa brand seat on my MTB and it's very comfortable. It has a slot down the middle and is well padded.

Read the thread on high rise handlebars-I have a post about the seat there.