I was here for a conference, but ended up staying on for a short break afterwards. The hotel is great; very clean and comfortable. The staff are extremely friendly and all seem to be having quite a of of fun while they're working - which is nice to see! Food is great - especially the endless breakfast. The only (small) thing is no tea/coffee making facilities in the room.
The hotel's about an hour's walk from the old town of Madrid, in a fairly business/finance orientated district. That said, if you look hard enough, there are enough places nearby to have a drink, or a nice meal, particularly in the streets behind the hotel. There's a Metro station nearby too, and it's all pretty easy to figure out.
I really enjoyed myself at Melia Castilia.