As funny as you'd expect, which is to say pretty damn funny. Partridge remains one of the most fully developed and endlessly fascinating comic creations of recent years, in my opinion, and none of that fascination wanes over the course of 300 pages of autobiography, even if some of it does go over old ground already covered in his various TV and radio shows. Some reviews suggest it also works as a satire on celebrity memoirs, but I wouldn't know as I've never read any. You probably already know if you'll enjoy it or not based on how amusing you find his incarnations in other media. Hard to imagine anyone picking this up who doesn't have at least a passing familiarity with him, although sometimes having seen certain episodes gives you an advantage. The "Smell the cheese!" incident related here from Alan's point of view, for example, is far funnier once you've seen the more "objective" presentation on TV in "I'm Alan Partridge".
One thought: this may be one of the few cases where the audio book is superior, as hearing Partridge narrate with his own inimitable cadence and inflections would inevitably improve on reading it in the Partridge voice in my head.