Games designed for the gamepad do work nicely on a gamepad, but they achieve this by slowing down the FPS gameplay so thoroughly that they functionally exist within their own genre, as well as providing substantial auto-aim. The PC-style of FPS gaming, which is substantially faster-paced than the console-style, is dependent on having a pointing-device for aiming.
Experiments in allowing gamepad and mouse players to compete directly have proven absurd to balance, since you either give the joystick players aim-bots or let them be crippled. Joysticks are meant to provide you with good control over a vector - you can comfortably select and gradually alter the velocity of something with a joystick.
Pointing devices are meant to control a position, not a vector. "facing", from a user perspective, is best expressed as a position. The player is thinking "I want to face this way" not "I want to turn this fast".
The main reason Halo and GoW feel bad on mouse+keyboard is that they feel glacially, frustratingly slow on a system where the conventions tend to be far faster-paced (because the interface allows for it) and because the games used heavy auto-aim to compensate for the difficulty in making precise movements on a gamepad.
Which is fine, I'm no snob. But at this point you've created a substantially different game with a substantially different experience.
edit: to take it further, notice the huge difference in the DOTA and RTS genres between PCs and consoles - places where auto-aim won't really fix the game. The interface has to be so fundamentally altered that they're a whole new genre.