Chuck, the M17 was a good piece of gear 20+ years ago. The last of these masks were produced in the 80's. Even the newest ones are well past expiration and new in-date filters are made of unobtainium. I've seen lots of M17's out there with cracks in the face pieces, rotten head harnesses, leaking eye pieces or eye pieces that are beginning to go yellow and crazed.
A lot of this has to do with how the mask has been stored, of course, but the fact is most of the M17s one encounters "in the wild" are just plain worn out (as are many, many other mask types as well, the M17 is just the one we are speaking about here)
Leaving age and filter availability aside for a bit, The M17 is something of a pain to change the filters in, requiring the complete removal of the mask and disassembly of the interior of the mask to access the filters. The filters are retained by a pair of very small "buttons" that hold the rubber closed over the filter. I've seen the holes for these buttons elongated out and even torn out completely. The filter protrudes through the hole in the side of the mask and requires some force to snap the exterior retainer on. Then one must reassemble the mask and replace it.
I don't doubt that the army has millions of masks in storage. Heck, they even issued M17 masks to dependents up near the DMZ in Korea in 99-2000. Operating on the theory that any mask is better than no mask, this makes a good point.
My point is, however good the M17/M17A1 was in it's day, that day has passed. Please note I did NOT recommend this mask for any kind of ABC/CBRN protection, only as a really glorified dust mask or MAYBE (and that a qualified maybe) for CS gas. Also please note I did point out some nice features like vision and the ability to wear corrective lenses.
Incidentally, if you are aware of filters for the M17 series that are rated for Ammonia, I'd like to know about them. I know that the M13A1 filter type is not rated for ammonia exposure, and I believe that they do not stop chlorine either, although I need to check that.
My overall point is that the M17 series and copies thereof, although cutting edge 20+ years ago, have been surpassed by the newer designs such as the M40 and MCU-2/P mask. These masks, and some foreign masks like the Korean K1, the British S10, and the Israeli M15 to name a few, eclipse the older M17 design in several areas, including filter design and availability, weight, ease of maintenance and comfort.
If you have a good, solid M17A1 with good, in-date M13A2 filters you certainly aren't unprotected; far from it. (I'm not sure there are any in-date M13A2 filters left, as the last manufacture date I've seen was the late 90's).
The Czech M10M needs to be regarded as a great full face dust mask for working in the shop or yard or in areas where there is mold or other irritants. As a CBRN mask, it fails.