Del Diego is a Madrid classic, and for years it was almost the only place where you could get a good drink. However, I've heard many tales of disappointment. I think it's a generational issue: the way they do things seem to have remained the same since they opened over twenty years ago. You don't really feel any willingness to keep on moving, to learn new things. It's a bit stale, but they know how to make a classic drink (although if you ask for a old-fashioned, make sure to let them know you'd rather they don't fill it with soda).
Bottoms Up Havana Cocteles Ginger
Yes, I agree with you. It has for many years being almost the only place in Madrid where you can get a decent cocktail, and fortunately things are changing. The place is nice but the drinks are a little to sweet for my taste. It is though a good place to stop before having dinner.