As a fan of this series, I have always worried that the basic premise of this series would outlast its welcome, with characters becoming narow parodies of themselves. It could easily happen with Alan's neurosis, Charlie's womanising and self gratification as well as Jakes slightly annoying stupidity. In fact season five succesfully overcomes this challenge by revisting the shows core comedic values and throwing in some subtle reinvention.
The main example of this is the changing relationship between Jake and Alan. Jake has entered his moody teenager phase and while this is well handled so as not to irritate, it does serve to freshen up the material and also help create new situations for the comedy. It also changes Alan as a character, especially as he starts to seek approval from Charlie as his relationship with Jake changes, this produces classic episodes in "kinda like necrophilia" and "the leather gear is in the guest room". Another bonus is that some of the irritating features of the fourth series such as Charlie's tiredness and getting old are lost in favour of more subtle crafted comedy in which Charlie's shortfalls are handled with a really funny episode in "shoes, hats and pickle jar lids".
As ever there are some good guest appearances with this season featuring, Jenny McCarthy, Christina Moore, and Richard Kind. If you are a fan of the earlier seasons especially 2 and 3, you will be a fan of this series.