I never understood those mesh heads. What's wrong with system heads? It seems to be the part of the system body you could find the least fault with. The shape is fully customizable, and with lots of skins around, you can get pretty much whatever look you are going for. Maybe there is more detail on a mesh head, but... well, how much detail do you need, really? A close-up of that mole on your cheek? Or of those tiny pimples? In terms of the overall detail of computer games in general (and SL in general as well), it really makes little sense to have that little bit extra resolution/detail on the face. The normal head offers plenty of detail unless you really want to zoom in really closely. At which point it becomes more clear that it is really a 3D drawing. But hey, that is what it is, really. Who zooms in that much on someone's face, anyway?
Plus, mesh-heads tend to look ugly.
Male mesh-heads always look the same, making everyone look like that one actor's younger brother. As Tantos said, loads of Bruce Willis's in SL, right now. Sure, some of the faces look good by themselves, but whatever character they have gets lost when you see your fourth looking exactly the same. And that is with the not-so-ugly mesh heads. And don't get me started about seeing the same afro guy walking around, seeing him standing on various places in the same sim simultaneously, wearing different outfits, but having recognizably THE SAME face.
Female mesh heads... aren't great either. Every female player seems to get them, these days, but often, it isn't an improvement at all, in terms of attractiveness. Loads of them look the same. Sure, there may be different models of Catwa heads, but they still look the same, pretty much. Loads of female avatars that used to look great now have that slightly creepy Uncanny valley look. Loads of girls look like they are 14 years old, now, because that is a common look with many of these mesh-heads. Loads have Jay Lenno-like massive chins, because of that LeLutka mesh head's shape. Many have that bloated botox lips look. Mesh heads of one creator nearly always have the same overall vibe. Some avi's using them look alright, I suppose, but for the most part, they tend to look creepy, rather than attractive. Sure... these heads look more realistic, up close. But if you zoom out a little, often, the proportions seem a bit out of whack. Mesh heads tend to look too large compared to the body below. Like big-head dolls. If you look at the pictures of the blogposting fashionistas (fashionistas tend to have skewed ideas about what is nice-looking, just as in RL), you get all these similar-looking doll-faces, showing off the mesh heads. Overall, it just tends to be... less nice. I get it. People change over because they can, and because everyone and their dog does it. But I am just surprised to see friends that had great avi's follow suit and hamper their avatars just because it is fashion.
From a pure 'visual RP' perspective, these mesh heads are a curse. They do take away more 'cartoon resolution' individuality and character and trade it in for staple, non-mod creepiness and disproportionality with more detail and a higher resolution. And just as with other computer games, anyone with sense and taste would prefer a more individualized, unique and character-full look over more zoomed-in detail and pixels per millimeter.
TL:DR? Want to look good? Use a mesh body, but stick with your system head. Screw those Bruce Willis, Justin Bieber and Ivanka Trump faces.