When he had a good script to work from (e.g. The Curse of Fenric). With poor scripts (e.g., Dragonfire) it was hard to watch.
The beauty of Tom Baker, and to a lesser extent, Jon Pertwee, was that they could carry the show over the bad storylines.
I agree with that 100%. Both had personalities that lent themselves well to good and bad scripts.
Have you seen a recent photo of Tom Baker? He has aged something fierce!