Yes, it's on my phone (siemens sx1) owned over 2 years now. But I play ogg files on the phone mostly, is has symbian operating system so I can put any program on it, even used it as navigator in my car with a gps mouse.
Now waiting for a Glacier mp3 player to arrive.

I currently have 3.... and they're great. Two are 1gb 1.5" with double speakers and headphone jacks, and the third is a 1.8" TFT. Unfortunately, I misplaced it, but hopefully I'll be able to find it soon

oh yes i do and i love them.

more features and value for money than iPods with due respect to them.

also websites like mympxplayers.org just put the icing on the cake with cool themes, useful tools and immediate and helpful support.

i have placed an order for another one for the 1.8 inch screen since my current one has a 1.5 inch one and also for the replaceable battery since i am not comfortable with soldering and stuff with the normal batteries in mp4 players :)