The stem is very important to me, although material is secondary to the shape. I clench quite a bit, so I prefer lighter straight pipes, or well-balanced bents and saddle mouthpieces -- the thinner and flatter the better. But I like Peterson p-lip stems too, as long as they're not the clumsily-made acrylic ones.
Only occasionally, however, has a stem been a deal breaker. A recent example was a gorgeous JT Cooke billiard with a steeply tapered stem. It was simply unclenchable for me. A good friend bought it instead and it's fine for him. I have two other JT Cooke pipes which clench beautifully for me, but they're saddle bits. His mouthpieces are a little on the thick side for my comfort, but I still love the pipes and the stem material, which Jim pours himself.
In descending order, these are my preferred stem materials: Juma, Polyester, horn, Bakelite, vulcanite, acrylic, plastic.
Juma and Polyester, like acrylic, do not oxidize, but are relatively softer and more comfortable. Horn is surprisingly comfortable, but soft enough that care must be taken not to wear it out. Bakelite is quite hard, but in the right hands (Nate King!) it can be fashioned into a fantasticly thin and comfortable mouthpiece. It will eventually oxidize, however. I like Vulcanite/Ebonite, but the maintenance is tiresome so I leave all my rubber-stemmed pipes in closed drawers, out of the light for protection from oxidation.