[mddw]

Of course you can. You can even use your own router.

[negrit]

No you can't use your own router instead of the freebox.

[demallien]

Well, you can, but you lose a whole bunch of important functionality, like being able to watch TV over IPTV (source - I work on SFR's STB, and we watch the competition closely :D)

[tonfa]

Well you can, but you'll have degraded performance and not access to tv/phone services.