I have to back up Elvira on this one: it really is the best place in Madrid right know (even though two new bars are closing the gap).
Elvira already commented on the design so I won't repeat what she said very well. I'd like to talk a bit more about their cocktail menu (all drinks at 11€, save for the Champagne ones). I do think it's one of the finest, better balanced menu in the city. Besides the obvious classics, they have a very interesting "Forgotten Classics" category -- cocktailians will know most of them obviously -- which helps casual patrons get acquainted with cocktails they might not have ordered if they were lost between the better known names: the Brooklyn, the Louisiana, the Remember the maine are such cocktails, usually very difficult to get properly done in Spain.
They also have a large selection of creations, from the Tangerine (Gin, St Germain, Tangerine juice, rosemary syrup), a very balanced drink perfect for your non-cocktailian friend that want a change from the Cosmo and the Mojito (consider it a gateway drug...), to the Avery (Tequila, Martini Bianco, Celery bitters and ginger) or the classic-style Don Alberto (Gin, Dubonnet, Vermouth dry & sweet).
But my favourite part of the menu is a selection of drinks brought back from travels all around the globe, cocktails tried in Boston, Alexandria or Amsterdam re-interpreted by the team. The Mr Antoni and the Trinidad Sour will probably ring some bells here; they have their own version on the menu.
The bartenders are all very nice and able, and VERY willing to go off-menu. One big difference with a place like Del Diego is that this place feels alive, the staff is always trying to learn and get better. They do not rest on their laurels.
Obviously, I don't like all their cocktails (I'm not very keen on their Mai Tai for instance) but it's a must try place. They opened a year ago, at the moment they're hugely successful. Better to visit monday to thursday or get there early -- they open at 4pm, not 7 as Elvira stated.
In the interest of full disclosure: I have no professional links with Le Cabrera, I just spend countless hours there (see my avatar for proof).
Bottoms Up Havana Cocteles Ginger
They've now opened a second place, a short walk from the first one, at Paseo de Recoletos, 2. It's located right in the centre of the city, on the very square where Real Madrid celebrate their triumphs (here's hoping they won't win anything this year).
They took over the restaurant / cafeteria part of Casa America (a cultural center dedicated to South America) early this month. The cocktail bar space (called "Le Cabrerita") is quite small and cosy but drinks are prepared (at the moment) at the back -- there's no mahogany -- although they are working on a sort of bar trolley. For cocktails in a classic -- and busier -- setting, I still recommend the main location, but this new place is very nice for quieter, more romantic dates.
The big advantage the new space has is the huge outside space the will be available as of May. Over 100 seats in the garden of the cultural centre, it will be very welcome over the summer.
However, for serious drinking, I still think the main address is the place to be.