The only airline that I fly on frequently enough to make membership of a FF scheme worthwhile is Emirates. Ive used Skywards points to upgrade from economy to business four times and there has always been availability although on one ocassion the upgrade only came through at the gate as I was about to board the aircraft. In conclusion FF membership has been good for me and as others have said, often the best use of points is to upgrade classes on long haul flights. The Skywards mall is pretty dire.
I note that Skywards points can now be used to pay for Easyjet flights. Has anyone used this and is it worthwhile? as Im thinking about a trip to Europe in the autumn. (Appreciate that Flyertalk may be a better place to ask though).