yes be very aware there is a problem ( do not have money/wallets in open accessable pockets ), travel light, also if you have luggage be aware to hold onto it, it is not unheard of for people to turn away and when they go to grab their luggage its gone.
Having said all that, dont be put off, many people go to Barcelona without a problem, it is a very beautiful city with lots of great sights to see. Just be mindful of your surroundings and your belongings and you should have no problems
Yes on the whole the metro is easy to use, the main tourist stations we used had good disabled access. If you look on the metro map it will show which stations have disabled access.


Try not to buy tickets for each journey
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they do a multi person journey travelcard. Thats one ticket for any number of people. We purchased a T10 which gave us ten journeys A one zone card is 7.85 euro and a two zone is 15.60 euro, for all the main sights we wanted ,we only needed a one zone, its much cheaper this way.
This link will help with disabled access questions and also travel tickets on another page
http://www.tmb.cat/en/transport-accessible
Hope this helps, don't be put off by the pickpocket issue, it's a lovely city, you just need to take care
margaret