While it's not high cinema, it's watchable, and certainly passes the time on a rainy afternoon. It could be said that Phileas' constant do-si-do with the Princess (as Passepartout puts it, "l'affaire du coeur") does drag a bit, but I can't find any reasons to be truly unhappy with it. It's more authentic than the Jackie Chan version (which I don't hate, either).
I don't see this portrayal of Fixx as a bumbler, either...efficient Fixx may be, but he was never a Nobel prize nominee, and Ustinov does well enough with what's provided. The Princess may have some anachronistic attitudes, but no historical movie has ever failed to cast the characters in at least a semi-modern mindset--it makes the characters more accessible to modern viewers. Of Eric Idle I'll say no more--I enjoy his work, and I don't care if the accent is ludicrous or not.
It's entirely possible that the novel simply can't be filmed. It wouldn't be the first one to have that happen. ("Hitchhiker's Guide to the Galaxy" comes to mind, for example.)