I don't really get what's the problem with the input system. Although haven't tried it myself, I've seen people using SFML for rendering and using OIS for input handling. OIS is engine/framework independent, you can even use it with text-mode applications if you want.
But, think about it. SFML already has its own input system, designed and developed by the same developer who did the library itself, so unless you need to use some really special input device (PS2/PS3/XBox(360)/Wii controllers, steering wheels, etc.), don't make your life harder and just use the input system that comes with SFML.