I go to Costa pretty regularly so I have some good insights into the experience of going to Costa.
The first thing (and the most important thing) is the coffee, overall at all locations I’ve been to the coffee is good, if you want a cortado you get a cortado, if you want a latte you get a latte, when compared with other coffee houses who are inconsistent in the strength and quality of their coffee, costa is the same at any franchise. (franchises could be a post in itself)

Compared to the competition
Costa and Starbucks (in the UK) are on equal footing IMO, so it’s purely personal preference for where you want to get you coffee from, but for me Costa is the definite winner.
The different franchises all have unique buildings (or new builds, like the one near me) and clean, well presented interiors, compared with Starbucks which often has sparse decorations in their franchisees, and tend to be in pre-made buildings that aren’t that inviting.
And comparing either to Cafe Nero just isn’t fair to Nero because the last time I went there was over 6 years ago lmao.

Favorite Drink?
I recently tried a Vanilla Latte when they got my order wrong, but it was actually pretty good so that’s in the S+ tier of coffee, the next best is the hot chocolate, plain but gets the job done and my second go to if I’m tired and don’t want to try anything new. And the best drink is probably the chocolate fudge brownie frappe, or just frappes in general because their a meal in themselves so you don’t pay for overpriced M&S food they sell.

The Food
To be fair on M&S food they do put discounts on them when no one buys them, but the same cannot be said for the cakes. The cakes make up the other side of why Costa is my preferred food there.

Overall
While it is expensive to go regularly you get quite a bit for the price, for example: as a student you can buy a single drink for £3.50ish and them stay for as long as you want (I usually stay until closing time), you also get access to clean toilets, free internet, and a table to yourself.

Yes I wrote this a while ago and forgot my train of thought but here we are I hope you enjoyed. I wrote the SSD post and finished this while in a class i don’t even take. (Picture from Costa in Skipton WY)