I use the 3M Powerflow PAPR.
http://www.envirosafetyproducts.com/3m-powerflow-respirator-3m-papr.html Pros:
-Uses a HEPA filter (basically N100)
-Full face mask, so eye protection is included, and I think there's a kit for glasses also
-Air pushes out through any breaks in the seal, so there's no chance of anything floating in, even when you're inhaling
-Doesn't put stress on your lungs and heart, so you can wear it for long periods of time
-The airflow on your face feels pretty nice in a hot attic
Cons:
-It's pretty bulky, especially with a huge filter on just one side
-As mentioned before, they are loud
-You have to wear the battery somewhere and have a cable running from the mask to the battery
-They don't make OV cartridges for it, so even though it filters out 99.97% of particulates 0.3 micrometers and larger, it doesn't do anything for the smell. . .
For what it's worth, I think this mask is great for cleanups, but it's too big and takes too long to throw on for anything else.