My son traveled to Europe last winter and stayed at a number
of hostels. He was in Paris on two separate occasions. The
first time he stayed at a hostel that was in the eastern
part of the city -- the biggest hostel in France, but I
can't remember the name -- and he really didn't like it. It
was too far out and he didn't care for the clientele. When
he returned a couple of weeks later, he stayed at the Young
and Happy on rue Mouffetard and really liked it, although it
gets, at best, mixed reviews in Trip Advisor.
He also stayed at two different places in London -- the IES Student Residence (Manresa Road at Kings Road, SW3 6NA Chelsea) which is open to the public only during school vacations, and Lancaster Hall Hotel. Lancaster Hall is part of the German YMCA and has a barebones youth wing. He liked both places. Lancaster Hall was particularly convenient because it's right near Paddington Station and he was able to catch an early train to Heathrow.
In Stockholm he stayed at City BackPackers Hostel http://www.hostels.com/en/availability.php/HostelNumber.1190 which is right near ther railroad station and he said it was clean, safe and convenient.
In Barcelona he stayed at the Gothic Point hostel which is right in the old part of the city. He said it was so good that even I might like it! Their website is http://www.gothicpoint.com but I haven't been able to get beyond their flash screens.
Landra in upstate NY