Are helmets with chin guards safer than those without?
I wish they had them 20 years ago when I was much younger and rode much more difficult terrain, with much less respect (aka fear) than I now have. (I ride mostly technical XC, not downhill).

As far a protection goes, if the choice is a traditional light weight helmet or one like this, you will be better off with this. It is not an alternate to a down hill helmet. 

Negative internet commentary on these tends to come from Downhill riders who do not believe they offer any real protection. As far as down hill riding, they are correct. They also tend to be good enough not to fall off on easy ground, and know how to fall if they do, so see no benefit over a light weight helmet.

However, I have seen some pretty horrendous injuries (google "Bicycle face plant") from riders face planting on paved and easy gravel roads. Most times its inexperienced riders doing silly things, rarely an experienced rider getting taken out by someone/something. These helmets are ideal for a XC rider who wants to keep his teeth and pretty looks, but is not prepared to spend hours suffocating under a heavy downhill helmet. 

Will it protect you from a face plant - For XC and road riding, face plants injure the face, its messy, painful and scars last a lifetime, as does the loss of teeth. Your face will be protected. 

Risk of chin guard snapping - at that point it would have been your jaw.... if you expect to fall off that hard, get a down hill helmet. If it does snap off, it has certainly done its job.