Thread count is no longer a reliable indicator of fabric quality. High thread count is simply a numbers game: many firms cheat the figures by using double twist yarns in the less expensive fabric. However, if you want a soft, warm bed, a superb elastic jersey sheet can be your cup of tea.
However, despite its pros — including the inherent water-resistance — polyester absorbs oil and grease and will leave you with the nightmare of trying to remove stains, particularly with white sheets.