I try to go to a decent independent whenever possible for coffee (of which there are plenty of gems in London).
As far as chains go, I tend to prefer Costa and Cafe Nero over Starbucks. I think a lot of people just love the Starbucks brand and think they do amazing coffee, when they don't, which is also why it has that poser culture in store.
Ultimately though, I think the Barista and by extension the individual franchise comes into it. I've had good coffees from Cafe Nero for example, but there are times when the Barista has burnt the milk, meaning you don't get a very good coffee.
Edit:
I've had coffee from Pret (usually when I've had to grab breakfast at work) and they can be quite nice, as they make their lattes pretty strong.
The one thing I will give Starbucks credit for is their chilled drinks, which is also what I suppose helped their popularity when expanding abroad. I don't have them very often, but the Strawberry and Cream Frappaucino is pretty good as a treat. I used the loyalty card points I had for a similar Frappaucino in Nero and it was terrible.