Firstly be aware that, when brand new, these can shed off synthetic material into your wort, so wash them first to prevent this. Secondly, as others have mentioned, tiny hop and/or grain material will go through the mesh holes. Take that for what it's worth. Thirdly, and most importantly, be VERY CAREFUL not to allow these bags to touch the hot bottom of the wort pot while steeping. If these contact the hot bottom of the pot, they will melt/burn. The burn results in a nasty gunk sticking to the bottom which will contaminate the wort. The bag will then burst open, spilling the grains or hops or whatever into the wort. CAREFUL!!!! However, they still work really well with all this considered! Can re-use a couple times, too (only need one for a whole batch).
I bought some of these a long time ago and I've been waiting to find something they are useful for. Pellet hops simply pass through the bag beacuse the weave is too open. At the end of a boil, there'll be virtually nothing left in the bag. If you are considering using these for pellet hops, just throw the hops in and forget the bag.
I have some that I've used for a long time that came from kits I bought, I'm not sure from where any more. Those are more of a closed weave and hold all the hops in. But mine are getting old and I wanted new ones. The search is still on.
These bags are great for spices or steeping specialty grains. They're so cheap, it makes sense to always buy a few extras.
While the product is exactly as described, I feel that the 15" length is a little on the small side for how I like to use the bags. I like to have extra length to after I tie it off, there's a length of bag to attach to the outside of the brew kettle. I guess I should stop complaining and just split my grain bill between multiple bags. (Easy to manage at such a cheap price)
These are great for specialty grains used in an extract recipe. When done, I throw the whole bag into the composter. No cleaning, no hassle. For pellet hops, better to use a nylon bag with a tighter weave.