I usually find the trade off of detail for jaggies worthwhile but, having now done a comparison with and without FXAA with XenoVerse which doesn't have many highly detailed textures, I think I might keep it on. Here's a comparison.
Downsampled 4K, 16x AF, no FXAA (same one posted earlier):
Downsampled 4K, 16x AF, in-game FXAA:
Inspecting very closely, there are many differences in jaggy amount, such as some clean up in my character's cape and hair, the pedestal for Mr. Satan is smoothed out, the snake road on the right is smooth, and most improved are the tree leaves. The only downsides I can see in this shot are on the objects with any texture detail, those being the bushes around the Mr. Satan statue, the TV displays, and the snake road scales. Most games have much more texture detail than XenoVerse such that turning on FXAA ends up blurring a ton of objects but, here, not so much of an issue. It definitely has a softer look which I normally don't like but, for this game, I think I'll keep it on.