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Of course, I use quantum chemistry programs. Believe me, you can do a lot more interesting and bigger things with 64-bit compared to 32-bit. And the programs are much faster as the blokes who code these babies use every iota of register if they can.
And, I suppose, I'm colored by my main 64-bit exposure being on my Alpha XP1000 and Digital/Compaq/HP's F90 compiler. Oooh...so nice, so well made. But, I'm guessing Opteron will be my next...then it'll be gfortran (someday) or pgf90 for me.