I use it all the time (all film, all printing) and it's pretty good. Particularly when you want to store a bunch of subtly different process variations, e.g. push/pull/rotary/inversion/stand, etc.
My main problem with it is that you can't enter a fully-customisable process (i.e. arbitrary list of named steps), which means that you can't time an E6 process without having to drop out to an external timer for the wash steps and need to remember when to do that. It's fine for B&W and any colour-neg process though if you abuse it slightly, e.g. enter the Bleach and Fix times as Fixer A and Fixer B, etc. And if you're using it for printing (I do my paper in a Jobo) and hook the iPhone up to your darkroom speakers, there's no way of turning the audible notification down or off without muting the iPhone's music output so you get annoying bell sounds over the music.
You do end up with a couple drops of water on the screen but as far as I can tell, that's harmless. I just wipe it off with a sleeve after each process.
The red 'safe' mode is not paper-safe! Not the app's fault though; a phone LCD is just not designed to have sufficient attenuation for that purpose.