Agree with everyone who said Starbucks/Costa etc stopped being 'cool' and a fad at least 20 years ago. They are now ubiquitous, on every high street, you see everyone from teens to parents, businessmen to pensioners in them.
People aren't going in to look cool, or play into a certain lifestyle. They are going because they are thirsty, fancy a drink, somewhere to sit, get out of the rain, or meet a friend.
I don't actually think they are particularly bad value. Honestly, where else can you go to meet a friend or sit for a few hours for less than a fiver? Restaurants would expect you to buy something, libraries are free but you can't always eat/talk loudly/find a good seat, McDonald's or somewhere may be slightly cheaper but not exactly a nice ambience to relax.
I am shocked by people who buy it every day, as that adds up to at least £20 a week, but as an occasional treat, it's really not expensive. I don't really like the plain coffee, but for all the other things (frappes, flavoured hot chocs etc), they are not actually more expensive than the alternative. With Frappe's for examples, you would think Mc Donalds would be the cheapest bog standard end of the market, but their frappes are only about 60p cheaper than costas, and they don't offer any of the alternatives (soya milk, low fat, different flavourings etc). To be honest, even if you made it at home you would have to buy the coffee, syrup flavouring, whipped cream, syrup, a blender....
Although I will moan that I hate Starbucks' ridiculously slow service. I have left the shop so many times because I've been queuing for ages and still not got anywhere. That does irritate me, but it's obviously not a problem for them as they're going strong.