I do think that spending money on balsamic vinegar is worth it. I normally use balsamic in salad dressing, meaning you get the full flavour of the vinegar; therefore I usually get a mid-range* bottle – at least – to get a better flavour. You can get a mid-range vinegar at at Tesco, and better quality vinegar at any number of delis and specialists.
Cheaper, younger balsamic vinegar can be sour, but it can be fine for using in cooking. The older balsamic vinegar gets the sweeter the taste – to the extent that you can drink it right out of the bottle! Of course, sour balsamic vinegar can be sweetened by adding sugar – but it’s not nearly so good as more mature, sweeter balsamic vinegar.
When I was shopping the other day I noticed that Tesco now have two mid-range vinegars for sale: their own aromatic aged vinegar; and Saturnia Gia. I did a wee taste test and they tasted pretty similar, to be honest! But the Saturnia Gia has the funkiest bottle, if you’re easily swayed.
* Balsamic vinegar seems to come in three price brackets: very low, a few £ per 250ml; mid-range, £9-10 per 250ml; and pricey.