Kabul is — like Cat's in Madrid — well-known for being one of the best hostels in Barcelona. And as such — also like Cat's — I took no chances, and booked my stay at this hostel well in advance. Kabul may have a rather unfortunate name, but otherwise it's a super-duper place. The bar and lounge / eating area, which takes up the entire first floor of the hostel, is its beating heart: it's packed with a ripper of a crowd every single evening (and buzzing during the day as well); it's filled with extra little attractions such as a pool and a fussball table; and it's where you can take advantage of most of the hostel's numerous freebies, such as the meals and the Internet. If you're able to book ahead for your stay in Barcelona, then take my word: don't even consider staying anywhere else, 'coz it don't get better than Kabul.
Rooms: a bit cramped — and they charge extra for sheets — but reasonably clean and quiet.
People: a crowd as international as that of the rest of Barcelona — the people here really are from every corner of the globe; although Brazilians seem to feature most strongly. Regardless of nationality, though, almost everyone here is young, friendly, and ready to party.
Staff: much like Cat's, they're a bit slow and incompetent at checking-in and checking-out their guests (maybe it's just a Spanish thing?). Otherwise great, and available 24/7.
Location: overlooking Barcelona's famous Plaça Reial, just off Las Ramblas, and walking- or metro-distance from most of the city's main sights.
Food: the hostel is famous for providing FREE DINNER EVERY NIGHT — and while this is a great bonus, don't expect too much, because the free dinner involves queueing for ½ an hour, and receiving a tiny plate of microwaved pasta or potatoes. The (also-free) all-you-can-eat buffet breakfast is, in my opinion, a much bigger highlight — here in Barcelona, where food in particular is an utter ripoff, I recommend pigging out on the breakfast here, and making it your main meal of the day.
Internet: about 4 PCs that are free for 20 minutes; but they're very busy and difficult to get hold of. If you need more time, the usual €1/hr immigrant-run cafes can be found about 10 minutes' walk away.
Hot water: nice and hot, although the standard very-annoying "water saving" taps are dodgy.