Have done this pub crawl many times over the past few years when visiting Madrid and cannot recommend it highly enough. It's well worth paying for as you save so much money on the drinks and entry to bars & clubs. There's usually a great mix of ages and nationalities on the tour, and the guides make sure that people are introduced and get chatting to each other. They also take you to decent, popular bars and clubs, not just the empty, unpopular ones that other pub crawls go to. My only warning would be not to drink before this pub crawl - things in Madrid start and finish very late and if you're drunk by 11pm you'll miss all the fun.
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