I bottle at least half of each batch in 2.25 L PET (they work best if you keep their original lid with them - thanks BribieG!). I don't brew many styles that need to be 'aged' much beyond carbonation (although I brew plenty that will 'take it' and demonstrate complimentary characteristics, i.e. dark ales and saisons). I also bottle in glass and have a couple of 9L corny kegs.
Admittedly I do drink quite a lot of beer, and 4 and a bit pints of a decent beer is pretty easy to consume, thus the larger bottles suit me well. They hold carbonation, can be decanted (depending on yeast variety), are a cinch to rinse and sanitise. It may not feel right Carnie, but we do brew in plastic after-all haha.
I notice no difference in beer deterioration in the PET (although any beer of mine is lucky to last beyond 3 months unless it has been purpose-brewed to do so) as compared to the other mediums and would suggest that in some cases, staling in PET may have to do with other beer handling techniques.
Certainly glass etc etc etc is considered the best conventional option if you feel your beers need plenty of weeks to become drinkable/peak (kegs aside - that would be 'o/t' haha). As with many people, I find many styles of AG beers to be at their best when fresh. Real ale is a classic example of this (if you make the mistake of thinking that a well travelled, aged and pasteurised bottled version of a pommy beer is a 'real ale', think again).
Also, (in my experience) most beers that you want the hops to be brash and obtrusive are best when fresh - although this may be a personal brewing issue as I recently found an 18 month old APA that was fantastic (bittered at 60 mins and flameout), and a 12 month old '10 min APA' that by comparison to the former had very little hop character even though it had over 3 times the amount of hops as the other one...
Again, each to their own etc etc etc - just my 2c. :icon_cheers: