Rockbox is an open source firmware that is is supported for a variety of portable players.

Does anyone have experience using this? What are the advantages installing this over using the stock firmware (which can be updated) on players?

http://www.rockbox.org/

I think the biggest advantage is that you can do FLAC on the iPod. I've seen pix of the previous GUIs and definitely not as clean as Apple's. But if FLAC is your thing, it's the only option for iPods.

When it comes to lossless I do choose FLAC but player support for FLAC is not an issue for me. Just out of curiosity will non Apple players be able to take advantage of Apples lossless codec under Rockbox?

Ah, good question. You know, I never heard of anyone going in that direction. Curious what you find out.