I played my most recent playthrough on my Android phone and was even able to use the same memory card files on the PC as well (through a PC PSX emulator).
It's excellent because I was able to play through the game 15 minutes at a time whenever I had some free time here and there during the day. FPse is currently $4.24. I can spend more than that during a trip to McDonalds. So for me, it was worth it.
That being said, it needs a really powerful and up-to-date phone to be able to run PSX games at realtime. And even then, the frame-limiting isn't perfect. You'll notice accuracy issues with quickly-repeating notes on the music, for example--they're just not as smooth as on the real PSX. On a playability scale from 1-10, it's an 8. Very playable with only a few minor issues.
(I've noticed the timing issues on ePSXe as well when using GPU plugins other than software renderers, so I can't really fault FPse)