I use rockwool. Well, technically, I use a rockwool based mix, with other components like sponge rock, charcoal, orchiata, leca, and other stuff. I basically mix in whatever's available and cost effective. I think other folks like the Fishers (i.e. OrchidWeb) use a straight rockwool mix for small growing Phrags and Phrag seedlings.
For Phrags, I go heavier on the rockwool. For Paphs, I go a little lighter on it. Parvies love it. Seedlings love it. Smaller sized plants love it.
I first started using it because it was the most cost effective potting mix for me. I was having to get potting mix shipped to me, and rockwool is so lightweight that once I factored in shipping costs, it was the least expensive potting media by volume (per cubic foot). Eventually, I was growing most of the slippers in rockwool.
However, more recently I've started experimenting more with other potting mixes and ingredients, so I've reduced the number of plants I grow in rockwool. Part of the reason is that as my collection has started to age, I have a number of big plants that I don't necessarily want to divide. I find it harder to properly water giant pots full of rockwool. The other issue I encountered is that I think I got a bad batch several years ago. It seemed like nothing would grow in the stuff, plants struggled after repotting, and root systems failed to thrive. I'm unsure why, since pH and TDS were good. I'll note that bad batch syndrome happens with pretty much any potting media from time to time. You'll get bark that rots to mush in less than a year, leca that crumbles to mud, sphag that rots within months, etc. Even so, it's made me nervous to rely solely on rockwool, so I'm basically hedging my bets.
Overall, I think the pros and cons are...
Pros: It's easily available locally (for me). If it's not available locally, it's still one of the most cost effective potting mixes once you factor in shipping costs because it's so lightweight. It holds moisture while still providing lots of air space. It's easy to tell when to water because the weight difference between wet and dry rockwool is very noticeable. It doesn't rot or break down, so you can go longer without repotting. Paphs and Phrags love it.
Cons: It must be properly prepared before use, soaking and rinsing is required, and it may need pH adjustment. Regular flushing with pure water is required since it's prone to mineral/nutrient build-up. Careful fertilization is important because of the mineral build-up concerns but also because rockwool doesn't provide any nutrition to the plants they are more reliant on fertilizers and nutrient deficiencies can pop up quickly.
