@oonabe Headshot is just like any other tool, whereas the quality of the results are in direct proportion to the time a user spends learning how to use said tool in order to achieve the best results. I have a lot of characters in game and animation projects I am working on, that are based on various actors and films I hold in high regard. Granted, while I would always prefer to simply create a likeness inside of Zbrush for far more detailed results, closer to the source material, Headshot is a more than viable alternative for someone who might be just starting out, or someone who doesn't want to spend a month or more perfecting each character design. I also agree with you that using the plugin straight out of the box does produce some irregular skin textures, the fact is that humans themselves have some pretty irregular skin textures, so it's just a matter of tweaking the maps a bit in order to get as close to reality as humanly possible.
I can only speak from personal experience, but the way I like to go about it is as follows: First, make sure the photo being used is a pretty high quality, preferably centered in a square space around 2160x2160. If you don't have a large, HQ photo, you can increase size and resolution, clean up any pixel noise, as well as center, etc., in Photoshop. If you don't have Photoshop, you can use the visible grid function in GIMP, since it is freeware. You make sure to get the bridge of the nose as aligned as close to the center of the grid as humanly possible, keeping in mid that no photo is perfectly symmetrical. Then after you import the picture into the plugin and generate the character and skin texture, it is CRUCIAL that you do this next step BEFORE you activate the SkinGen editor, as doing this afterwards will give you wonky results. What you do is select your character in the scene tab, then head over your texture list and select the head, then exporting ans saving the diffuse map for the face/head to an external folder. Once you have the face diffuse map saved, you then open up the saved copy in your edito of choice, i.e. Photoshop, or GIMP. You create a grid overlay and expand the size and alignment so that there is a perfect center line aligned with your dimensions. The generated map will be 4K so keep that in mind. What I do then is use the rectangle select and select the half of the grid that has the least flaws and best details, then copy and paste as a new layer. Once you have your new layer, you flip horizontal and move into position so that it lines up perfectly with the opposite side of the face, thus giving you symmetrical details. The next step, you merge down the new half (after it is perfectly aligned of course) and then use whatever tools you need to clean up any extreme skin discoloration produced as a photo effect, i.e. excessive highlighting or shadows in the photo that don't translate well to skin textures, as well as the darkened sections on the eyelids generated from eyelashes in the photos, etc. The goal being to get the skin blended as best as humanly possible. Once you are remotely happy with the cleanup, you then export the finished map and then drag it back into the facial diffuse slot in your texture section. This way, once you go to finally activate your SkinGen editor, you will get a prompt telling you that an edit has been made and asking you if you would like to keep the results. Click "Yes". Now, when the SkinGen editor, activates, it will generate an overall skin texture reflective of your polished results. Now you can tweak any adjustments you need within the SkinGen editor itself and get much better results.
There are plenty of tutorials online detailing similar techniques, many offered by Reallusion themselves, or many of the experts who have spent enough time with the software to get their results pretty polished.
Like I said, I can only speak from personal experience, but here are some the results of my character designs and renders from models generated using Headshot and the techniques listed above....







artstation.com/dreadmoormediaarts