I voted all cycles and all supplements forever, although I also view them on a case-by-case basis. As long as the book quality is good and I have a use for them, I will purchase them. It's unfortunate that there isn't an option for "will pick up most supplements, as long as the quality stays high" or the like.
PHBs are useful when I need to check rules or look up someone's power at the table. I don't see any particular reason to limit the number of them I purchase, although if one was put out that didn't have any classes I was interested in, then I might pass on it.
Both DMGs have had excellent advice, and also useful information on monster creation and templates. As long as they keep putting out interesting material and good advice, I'll keep picking them up. Even for advice I have already heard before, it can be helpful to be reminded of the basics to help refocus my efforts in some areas.
The MMs I get mostly for the pictures to show the players what a monster looks like, and for the occasional times I need to improvise quick encounters at the table. As I do most of my encounter planning via DDI, I might decide not to get MMs in the future.
The X power supplements can be fun to look through, and helpful for new players. Although the character builder and DDI have all the crunch, I still enjoy some of the flavor in the supplements. It has helped me as a player thinking about characters and as a GM for adding things to my worlds.
As far as other supplements, I have not yet picked up AV2, though I might pick it and future AVs up. I am going to look at the Race supplements before I decide whether or not to get them. The same is true for the Location books. I haven't bought any WOTC adventures other than the Eberron specific one (my wife and I are huge Eberron fans). I haven't gotten anything for FR yet, but I have the Eberron PH and CG, and I will pick up Dark Sun.