+=I like Sundstrom a lot. Best particle filtration I've ever found and the battery driven hip pack on the SR500 makes things very comfortable. Drawback is the pricetag.
I have been working with that kind of gear for years, and I can point you another drawback. We used to call this kind of respirator to "fart fan" for obvious reasons. It is great way to recycle, if you have eaten something which gives you some gas