I played through the entirety of Witcher 1 using an Xbox controller with a Pinnacle profile last year. I found the custom controller layout very pleasant and well suited for the pad(though I made some very minor edits myself to the site's profile).
Really though, the best reason to use a 360 controller is for the analog stick movement. Witcher 1 spends most of the player's time meandering back and forth through countrysides, swamps, and large environs. Giving your 'w' key a rest in place of stick movement was worth the effort of getting the Pinnacle profiler. Just for a pure comfort level.
You will likely still keep your mouse close by for faster menu/inventory management(though I got plenty conditioned to using the right-stick instead over time). As for combat, I dug being able to quickly and efficiently switch stances and swords via the face buttons. Instead of tapping the mouse buttons in tune for sword combos, you now hit a trigger. Perfectly translatable. Combat is not really "improved" with a gamepad, but all the functionality translates really well(and rumble is a nice touch with the controller).
It takes a bit of conditioning to wrap your head around using the triggers so much(acting in place of a mouse click, they will initiate dialog and world interaction), but once you do, the stick movement and the snappy stance changes made it my preferred control method for Witcher 1.