Devised my itinerary weeks ago. No hiccups in planning until I tried to reserve a decent room near the Ramblas for a weekend in late September. Almost everything Rick recommends, and many others rated highly on Trip Adviser, were completely sold out. I grabbed the only room I could find - a junior suite - for 3 times what I wanted. Better than re-writing the last 10 days if my trip. Plus, it's just a reservation, and I can always trade down if I can discover a cancelation somewhere.
Anyway, out of 16 cities in Europe over a six week trip, this is the first trouble I've run into booking a room. I'm told it is simply high season, and a weekend as well.