Linen sheets are the finest bed clothes fabric there is. Exceptionally cool in summer due to linen's unique properties, and they should last a lifetime with proper care. The first time I went to Ireland I bought a set and they were promptly destroyed when my father, trying to be helpful, chucked them in the dryer. I just about had apoplexy because they cost me a little over $700. Linen IS expensive, but very worth it.
My winter sheets are German-made flannels and wonderfully cozy. It's tough to get out of bed in winter!