not at all. some people would say the same about london, but if you're sensible it's fine there too. going to places like that i leave my wallet in the glove compartment of the car or in a case back at the hotel and just take a bit of loose change, a credit card and a few notes, and put that somewhere sensible. basically, act like a tourist with a big rucksack, a map and a huge camera and you're going to get targeted!
personally i stay outside of the city for the racing and just go in for a nosey around on the monday after the race, then go to the airport monday night. there's a good train service from granollers into the city, and the tube network is obviously very good. if it's just you i'm not really sure it's worth spending more than a day there unless you're really into art galleries and stuff, but if you are there's plenty to do and see. there's a couple of nice parks to take a stroll in or sit down and read a book in the sun as well.
sparky's cautionary tale should remind us all to check any hotel "services" in case they include "something for the weekend"
