Las Ramblas at anytime is an amazing place but you have to keep your wits about you. I got my wallet stolen from my bag, which was on my person at the time, on a quiet sunday morning. The thiefs are not all Catalonian, many are professional gangs from all over Europe. Make sure your bag is zipped at all times, better still keep your money in a bum bag etc. This happens a lot in Barcelona as it does in most European cities, but don't let it spoil your trip. I fully intend to go back real soon. One further bit of advice, if you don't want to go to Barcelona when it is at its most hectic, then avoid the Merci Festival at the end of September. I had a great time but it is very, very busy.
Happy travels