I can't speak for the Roku device, but the Squeezebox supports MP3 and WAV natively. The server converts any other formats on the fly. So, for FLAC, it will convert it to WAV and stream it down to the Squeezebox. Obviously, since both FLAC and WAV are lossless, you will hear the music in its full sonic goodness.
I have a Squeezebox, and I highly recommend it. It is real easy to set-up and use and has a great interface. If you use iTunes (I use it on Mac OS X), it reads your library directly, so you only have one place you have to go to maintain your collection, which is very nice. I have no regrets about purchasing it. In fact, I'm thinking of getting a second one.