This may go without saying, but be sure to try on whatever you're interested in before buying. Different helmets have different shapes, as do different heads. Not all helmets will fit equally well. If a helmet puts too much pressure on your forehead, you'll end up hating it no matter how great the reviews are. Ditto if it's too narrow, and won't go down far enough. A helmet that's too loose won't be comfortable when the wind starts pushing it around, and won't protect as well as a properly fitted one.
I tried an FS-15 the other day, and as much as I'd like to buy one (seems like a great deal), it just doesn't fit my head. The medium is too loose, and moves from side to side if I shake my head. The small is too tight, especially on my forehead, and was starting to hurt after just a couple of minutes in the store.
Even different helmets from the same manufacturer vary. The IS-16, also from HJC, fits much better. Not perfect, but close enough I could live with it. Unfortunately, it's not Snell approved, and I don't know if the slot cut into the top of it for the inner tinted visor compromises safety more than I'm willing to risk.
The best fitting helmet (for me) is the Arai Profile. Even the similar Arai Vector is different enough in shape that it puts pressure on my forehead.