The CharacterController doesn't work in this case because it needs Y to be the normal direction. A good way to do this is to use a rigidbody (uncheck Use Gravity), and apply a local gravity in the form of a constant force opposite to the character normal direction - the character normal must be updated using a raycast to the down side.
This script does this. I created an empty object, added a box collider and a rigidbody (useGravity = false), then childed my model to it. The terrain normal is constantly determined with the downside raycast, and smoothly updates myNormal (the character normal). myNormal is used to create the local gravity, so the character is always being attracted to the surface under its feet. In order to align the character to myNormal without loosing its current forward direction, a smart trick is used (thanks to -T- for that!): the current forward is found from the cross product between transform.right and myNormal, and the desired rotation is calculated with LookRotation(new forward, myNormal) - this returns a rotation that keeps the character looking forward and with its head pointing in myNormal direction.The weight force is applied at FixedUpdate to make it strictly constant. The character moves with WS using Translate, and can even jump to its vertical direction.
ADDED FEATURE: When jump is pressed, the character casts a forward ray; if the ray hits any wall in the jumpRange distance, the character jumps and rotates nicelly to land in this wall - much like someone with magnetic boots in the outer space.

EDITED: The original algorithm used had a big problem when the character was fully upside down: it started to flip back/forth at random points, driving us crazy. Thanks to a Boo script suggested by -T-, the character now keeps its forward direction under all circumstances, and call walk on the roof or on spherical planets like expected.

var moveSpeed: float = 6; // move speed
var turnSpeed: float = 90; // turning speed (degrees/second)
var lerpSpeed: float = 10; // smoothing speed
var gravity: float = 10; // gravity acceleration
var isGrounded: boolean;
var deltaGround: float = 0.2; // character is grounded up to this distance
var jumpSpeed: float = 10; // vertical jump initial speed
var jumpRange: float = 10; // range to detect target wall

private var surfaceNormal: Vector3; // current surface normal
private var myNormal: Vector3; // character normal
private var distGround: float; // distance from character position to ground
private var jumping = false; // flag "I'm jumping to wall"
private var vertSpeed: float = 0; // vertical jump current speed 

function Start(){
    myNormal = transform.up; // normal starts as character up direction 
    rigidbody.freezeRotation = true; // disable physics rotation
    // distance from transform.position to ground
    distGround = collider.bounds.extents.y - collider.center.y;  
}

function FixedUpdate(){
    // apply constant weight force according to character normal:
    rigidbody.AddForce(-gravity*rigidbody.mass*myNormal);
}

function Update(){
    // jump code - jump to wall or simple jump
    if (jumping) return;  // abort Update while jumping to a wall
    var ray: Ray;
    var hit: RaycastHit;
    if (Input.GetButtonDown("Jump")){ // jump pressed:
        ray = Ray(transform.position, transform.forward);
        if (Physics.Raycast(ray, hit, jumpRange)){ // wall ahead?
            JumpToWall(hit.point, hit.normal); // yes: jump to the wall
        }
        else if (isGrounded){ // no: if grounded, jump up
            rigidbody.velocity += jumpSpeed * myNormal;
        }                
    }
    
    // movement code - turn left/right with Horizontal axis:
    transform.Rotate(0, Input.GetAxis("Horizontal")*turnSpeed*Time.deltaTime, 0);
    // update surface normal and isGrounded:
    ray = Ray(transform.position, -myNormal); // cast ray downwards
    if (Physics.Raycast(ray, hit)){ // use it to update myNormal and isGrounded
        isGrounded = hit.distance <= distGround + deltaGround;
        surfaceNormal = hit.normal;
    }
    else {
        isGrounded = false;
        // assume usual ground normal to avoid "falling forever"
        surfaceNormal = Vector3.up; 
    }
    myNormal = Vector3.Lerp(myNormal, surfaceNormal, lerpSpeed*Time.deltaTime);
    // find forward direction with new myNormal:
    var myForward = Vector3.Cross(transform.right, myNormal);
    // align character to the new myNormal while keeping the forward direction:
    var targetRot = Quaternion.LookRotation(myForward, myNormal);
    transform.rotation = Quaternion.Lerp(transform.rotation, targetRot, lerpSpeed*Time.deltaTime);
    // move the character forth/back with Vertical axis:
    transform.Translate(0, 0, Input.GetAxis("Vertical")*moveSpeed*Time.deltaTime); 
}

function JumpToWall(point: Vector3, normal: Vector3){
    // jump to wall 
    jumping = true; // signal it's jumping to wall
    rigidbody.isKinematic = true; // disable physics while jumping
    var orgPos = transform.position;
    var orgRot = transform.rotation;
    var dstPos = point + normal * (distGround + 0.5); // will jump to 0.5 above wall
    var myForward = Vector3.Cross(transform.right, normal);
    var dstRot = Quaternion.LookRotation(myForward, normal);
    for (var t: float = 0.0; t < 1.0; ){
        t += Time.deltaTime;
        transform.position = Vector3.Lerp(orgPos, dstPos, t);
        transform.rotation = Quaternion.Slerp(orgRot, dstRot, t);
        yield; // return here next frame
    }
    myNormal = normal; // update myNormal
    rigidbody.isKinematic = false; // enable physics
    jumping = false; // jumping to wall finished
}

i know one of my friends who made a spider game literally rotated the whole level, making the walls turn into floors. not sure if its the best way, but it looked good

There's another question that addresses the same issue here. I posted an answer to that one (two, actually, but one just points back here).

how come when i add packages i get a bunch of errors in my game and i cant play until i remove the addon i added the locomotive demmo for my controller and i didnt even apply it yet and i get a bunch of errors but the demo works fine seperatly and the locomotive addon is good for the gravity thing if you can get it to work

The demo of my game here: Download. I used this script