There's a chart that shows the distribution of impacts according to a handful of studies on a full face helmet. Replace whichever part you omit with your particular helmet, with the coinciding body part.
Personally, although half-shell helmets can be the difference between life and death for a lot of people, I wouldn't go without at least a 3/4 helmet. Personal preference, but I don't feel a half helmet provides adequate protection. (Beats the heck out of no helmet though!)
I actually find my full face very comfortable. It doesn't bother me in the heat (though it does others), and I love having it in the rain or cold! In the summer, when it's warm, I wear my visor up with sunglasses or safety goggles underneath (so I get tons of airflow), but if I start hitting a ton of dust getting kicked up (like a mower or construction or something like that) I can reach up and flip that visor down and be sealed away, it's not like that!
So, let's see, pro's and cons of a full face:
Easy to seal away rain and debris
Less wind noise than other types (if it's a decent quality full face, some FF helmets are not very aerodynamic and so they actually GENERATE wind noise)
Provide the best protection of any helmet. For a variety of reasons, for one, it provides more coverage, and additionally, they know their customers have safety in mind and tend to work very hard to build them out of materials and technologies proven to reduce injuries. For many, a half-shell helmet is like liability insurance... keeps ya legal, so some half shell manufacturers focus purely on looking cool and being comfortable for riders who would otherwise not wear a helmet. (Not always! But sometimes)
Can leave you with a sweaty head on a hot day
Cheaper FF helmets can be noisy, and poorly vented FF helmets can be hot
FF helmet fit is much more critical and difficult than other types in terms of comfort, it's harder to find a perfectly fitting FF, and it can be uncomfortable over time if it doesn't fit well (on that note, don't shop online, go in store, try it on, walk around for a few minutes with it on, etc.)
It's a matter of personal preference. Motorcycle gear is all about balancing safety and comfort. I know a guy who bought a full-face helmet because he wanted the BEST protection and said if it's not full face, it might as well not be a helmet. But when it gets hot he just doesn't wear it at all! He'd be better off with a 3/4 helmet he wore all the time! Same with gear. If you wear gear, get stuff you'll wear all the time. Leathers offer more protection over textile or mesh, but if half the time you ride you don't wear it.. what's the point?