CAS is generally much faster than locking, but it does depend on the degree of contention. Because CAS may force a retry if the value changes between reading and comparing, a thread can theoretically get stuck in a busy-wait if the variable in question is being hit hard by many other threads (or if it is expensive to compute a new value from the old value (or both)).
The main issue with CAS is that it is much more difficult to program with correctly than locking. Mind you, locking is, in turn, much harder to use correctly than message-passing or STM, so don’t take this as a ringing endorsement for the use of locks.