FranticFish - 7:56 am on Mar 29, 2013 (gmt 0)
Google indicate whether your <meta> descriptions are duplicate (and too long or too short) in Webmaster Tools, so this indicates to me that they consider them a quality signal on some level.
The bolding of search terms found in the tag gives reinforcement in the SERPs too, so it's worth writing one that includes the keywords for that page.
I don't know if Google compares them across different sites to see if they are unique, but I'd not take one from other site myself. On whatever level that they are a quality signal, they're an easy one to get right, so why not do that?