Hi, yeah I went last year, it was great! Beautiful weather, I got to explore Barcelona & of course F1 cars for 20€ (give or take, can’t remember exactly). I think a couple of the tribunes on the inside of the track were closed off, otherwise yeah, you could walk around all the grandstands as you pleased. I read some advice on the internet before hand that it was difficult to walk from the train station to the track – this is NOT true, and I would advise walking if you can get a map on your phone or whatever. It seemed there’s like 1 or 2 taxis in Montmelo, and it took quite a long time to get in it (it was going back and forth I presume). There’s no shuttles or anything like that for testing. I have an account of the day and some photos and videos on my blog if you’re interested: http://gwannelsandiego.blogspot.fr/2012/02/barcelona-formula-one-testing.html
Oh, and this may be obvious, but don’t forget that they only run one driver at a time, so if there’s someone you particularly want to see and you’re only going for one or two days, try to find a list of who’s driving on what day (probably not available until quite soon before testing starts, so I suppose it depends how flexible you are).