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Personally, I've just recently moved to XOOPS from Postnuke. What I liked about XOOPS where the extremely helpful and forthcoming community. I did look at several others, and since I don't consider myself to be a programmer (not a good one, anyway :)) the thing that attracted me to XOOPS where the easy install, the community and developers, and that it seemed easier to customize than most.
I think these qualities makes it stand out amongst the cms crowd. For me, as a non programmer at least, these were the factors that made me choose Xoops.