Just another wee +1 to the mention of the JSP Powercap IP.
I tried the Trend Airshield and found that with most of it's weight concentrated at the front, it gave me a sore and cramped neck after a short time, given that while I'm working, I'm normally looking downward, a bad position for the neck while it's supporting weight.
The JSP solution has the battery at the back of the cap, and also weighs a considerable amount less. I've found it very comfortable, very effective, and you can get simple paper inserts that go in front of the air filters, to take the worst of the bulk dust, and prolong the life of your main filters. The battery lasts a good few hours, though I've no idea how many it'll actually go for - as I tend to use the unit in bursts of an hour or two, with several days between, and just charge it when it sounds like it's getting a little slower.
I'd advise getting the plastic "screen protector" type film to go over the face plate on it as well, as it's a lot cheaper to replace than the visor, if you do have an accident that causes a scratch.
Love the unit myself, it may be less snazzy than some of the top end full respirator solutions, but it's damn near perfect for me, and no hoses or belt attachments. If it's light enough for my damaged and faulty neck, it'd be light enough for anyone!
Nic.