I have used FastStone Image Viewer for years. (And I'm assuming theat, when people say "FastStone", they are likely referring to the Image Viewer, even though there are other FastStone apps.)
I use FIV strictly for handling JPEGs (since I don't do RAW), and it does nearly everything I need to process JPEGs (as long as the changes are global - i.e., the changes apply to the whole image). Or,when I do have to make a change to just part of the image, I just hit "e", and up comes The Gimp (the editor I have configured to open with the "e" for "edit" command in FIV) -- I make my changes, save the file, and the image back in FIV is instantly updated to show the change. Easy-peasy.
I do have some minor gripes about FIV, mostly about the lack of certain (often fairly Windows-standard) keystroke combos, and the inability to be able to configure the app to accept new keystroke combos (or keystroke changes) that the user might like (in order to simplify his/her workflow). But, these are minor gripes (mostly...).
I do hope that regular users of FIV will opt to register it, of course...
[Registrations are "for life" - i.e., they cover all future version upgrades.]
By the way, FastStone MaxView is great for browsing the image files within compressed (ZIP or RAR) files without having to unzip (or unrar) the files first. And FastSone Capture is also a nice utility, for making all sorts of screen grabs.