I had the opportunity to spend the day up at the Porsche Experience Center in Los Angeles a few weeks ago, and ended up driving the 718 Cayman S and 911 GTS. The Cayman had an alcantara steering wheel, and it was the first car I had driven with one. Loved the feel, but have always hesitated about alcantara since it's difficult to clean and wears more quickly than leather, plus the leather on my wheel was in excellent condition.
I came across another thread on here where a local did a steering wheel exchange through Royal Steering Wheels and was really happy with the result, and the pricing was very reasonable, so I pulled the trigger! The pricing really alleviated my concerns about the wear, because if it wears out quickly I'll just get another one, or move back to leather again in the future.
Before
After
Very happy with the results!
Here's a link to some other example Z4 wheels:
http://royalsteeringwheels.com/bmw-z-series/
And a link which covers the material choices they offer:
http://royalsteeringwheels.com/offer/
Cheers!
-Rob