I use it once a week, at least. It's not especially helpful looking up CSRs of Union troops, as most of those have not been digitized. (All, or nearly all, USCTs have been.) All the Confederate CSRs are in there, I believe. The Soldiers and Sailors database gives the barest minimum of info on individuals, unless I'm not using it right.
Fold3 also has a lot of other collections that I've used, including citizens' files, pension records, Navy enlistment records, admiralty court records from Key West, and so on. And of course, it covers all U.S. conflicts. So, yeah, it's an important source for me. I'm a cheap SoB, and I cringe every time it comes up for renewal, but then I think about how often I use it and stick with it.
Added: All the Confederate CSRs are available digitally because many years ago (the 1950s or 60s), the United Daughters of the Confederacy gave the money to have them microfilmed. So what you see of them online at Fold3 is a digitization of that old microfilm. The Union states, by contrast, are did not have an outside sponsor so most of those aren't done, and have to be ordered directly from NARA.