I use the Trend Airshield Pro (2-3 months). I am happy with the way it works (but it is overpriced IMHO).
Noise: I can hear the fan running, but it is not load enough to be a bother. I can hear the radio or someone talking with it running.
Comfort: The headband is adjustable - there is a knob at the back that you turn to fit, then push in to lock. Unfortunately, I pick up the shield by the headband and sometimes pull the knob loose and have to re-fit. I have not found the "sweet spot" yet, so I re-tension the headband every couple of hours for comfort. I notice some neck strain after 3-4 hours of continuous wear, but not a bother if I take a break every now and then to take it off and rest my neck.
Usefulness: No fogging of shield (with the fan on), no problems with dust (wood or grinder) since I started using it. I have not had to change the filters or battery yet.
Issues: The only issue I have so far, is that the lower part of the shield that is made of some material to "seal" around the chin - does not clear my face when I raise the shield. It seems to hang right in front of my eyes. So, I have to take the shield off completely to see around it. Small price to pay...
I highly recommend this if you can afford it.