The only problem with buying pre-made liquid is that you have to buy a LOT of crap flavours before you can find one that works for you. I remember back in the day buying 10ml bottles at £5.00 a pop. Once you've spent a couple of hundred pounds on bog-standard one or two flavour mixes then DIY seems like a no-brainer.
And why do manufacturers call liquids things like 'the toad's gonads' - just list the feckin flavours.
Admittedly there are some liquids i can't replicate - i love donuts being a good example - but most are just a couple of fruits, some ethyl maltol and a bit of cream/custard/yoghurt etc.