Yes, "So Long and Thanks for All The Fish" starts out okay - slagging Rickmansworth is always a starter for ten - but degenerates fairly quickly. The first book - and parts of the second and third - were, however, timelessly hilarious.*
* The Hitch Hiker's Guide to the Galaxy, CK Edition, has this to say about timeless hilarity: 'Timeless hilarity is that state in which the diaphragm of someone laughing has contracted against the lungs in so semi-permanent a manner as to render the sufferer unable to (a) breath, or (b) remember what the dickens it was that he or she was laughing about. This is because the minute or so it takes for normal breathing to resume seems like hours, and sufferers often see what they perceive to be their entire incredibly drab and boring lives rushing before their eyes. (It is a curious coincidence that the majority of people who report this side-effect are staring out of an express train window at the exact minute it strikes.) This takes all of the fun out of the situation, every time.
'There are other, more extreme forms of timeless hilarity. cf "laughing till it hurts" and "laughing yourself to death".
'The Slannerdalath of Myxonas VIII were known to be prone to the timeless hilarity ailment, but rubbed along well enough until Zaphod Beeblebrox, then President of the Entire Galaxy, gave a lunch time speech on democracy. The entire population immediately succumbed to terminal timeless hilarity, and Beeblebrox had genocide added to his already incredibly long rap sheet.
'Timeless hilarity is therefore, ipso facto, not at all a laughing matter.'
