You're both right... I'd like to use something other than Blizz because I feel there are other UF that convey the information better (and include Party Targets). However, I don't feel the difference is that great and am therefore concerned about memory/cpu effects of adding a third-party UF addon. My take from Funky's post is that I shouldn't expect to see a whole lot of bad (or good) from using another UF. In an ideal world I would have hoped that using a UF without portraits for example would free up some cpu cycles but I guess it is truly minimal.
I tend to only add addons that will make my wow experience more efficient rather than make my interface better looking. As such I tend to revert to the Blizzard UF because I don't feel like the addon ones (be that Sage, ag_uf etc) add enough information to be worth it. My question is, could I actually improve the game performance by using a simple third party UF like Sage or ag_ug in place of the Blizzard ones?