Like a few comic actors, male and female, if he is kept on a tight leash by the director then he can be funny, bits of Anchorman have me in stitches. The moment he is given licence the laughter stops. Improvisation of scripts is rarely funny or interesting for the viewer, there is a reason why a scriptwriter is there.I avoid anything with Will Ferrell in it. I just can't stand the stuff he does.
The film critic Mark Kermode has a theory that the more fun the actors look like they are having on screen the less funny the film. Will Ferrell does seem to enjoy himself a lot in films