I do in two of the machines in my house at the moment (a dual P3-S with 2GB of PC133, and a P4 2.26 with 1.5GB of PC1066). Honestly, it doesn't help 90% of the time for me..... there are only a FEW applications where it makes a difference. But boy does it make a difference then--anything that will prevent swapping to the HDD is great.
Before you upgrade, just look at your RAM usage for a few days in Task Manager. If you're not going over your limit, then don't do it--spend your money elsewhere. I had a VERY specific use in mind when I added that much RAM to those machines.