It is rare that I read a book all the way through in a day, but I completed this one in a three hour car journey (not while driving though!) At 142 pages, it's an easy book to read and is like a literature version of the channel 4 series Fonejacker.
There are some good letters that were worth book marking, including the poem about aluminium set to 'Oh my Darling Clementine' and the letter to the Society of Existential Analysis. The letters also give you some good ideas for letter writing and can be light-heartedly amusing, steering clear of puerility.
However, this is where the similarity between the hype (on the cover) and the content ends. The book does seem like someone's hobby when bored, and the timewasting nature of the letters follow the tawdry cliche of being a waste of your (the readers) time.
It is reminiscent of reading one of those Clarkson books of magazine articles - essentially what you get is just a load of raw material, which could do with processing into something better. Maybe this is why the fawning praise is from TV comedians - it's good source material, but little more.
So, to conclude, if you are a comedian, then this is a good book for you, as it has much in the way of diverse and useful muse material. However, if you are after comedy then get a hold of some early Jeremy Clarkson or Douglas Adams. Better still, experiment a little by getting Nikolai Gogol's St Petersburg Tales, which are a lesson in just what can be made with comedic ideas...