I was actually preparing some documentation that explain Owl-specific Faust usage. It's mostly ready, so I can upload a link here. It's meant to describe next update, which means that a few features are not available yet as they are not merged to official branch of OwlLibrary repo that is in Rebeltech online compiler. Will try to finish it soon.
For now you should go through there official tutorials, since question like "how do I get audio pass through" are not really specific to Owl (btw, it's simply "process = _;")
Generally, Faust is in a better shape than PD. Performance is great (I've seen better results than hand-written C++ for some common DSP code). Faust went through some compiler changes over the years (switch to LLVM and a few version upgrades), but everything seems fairly stable for now. You may consider setting up local build environment if you're working on a linux machine - personally I find it easier to build locally rather than using online compiler.
Guitarix is great, but there's so much stuff in their official library, that I have no need to bother with anything else for a long time. Also, I'm pretty sure they've been merging to it some stuff originating from guitarix.