Glass demijohns are great if, like me, you were making wine 20+ years ago and still had them up in the loft, or if you get lucky on freecycle, or manage to spot any at car boot sales or in charity shops.
However, I'm not about to spend seven quid apiece on new ones.
5 litres of mineral water is a quid or less in most supermarkets, and the bottles can be re-used a few times.
The inside ridges can be a nuisance when clearing a wine, but just give the PET a quarter turn every now and then to dislodge the sediment.