I've been to both Barcelona (2009) & Jerez (2010) for testing & can throughly recommend it. It's a great way to practice your action technique all day if you want for a minimal entry fee (if a bit lonely!)
The ticket for Barca allows you access the outside of the whole track, but not all stands around the track are open. There are some good positions to shoot from and the variety of angles is great, but you are quite far from the tarmac in a lot of areas. I had a 300mm on a crop body and wished I could have greater reach. The main grandstand is covered though and at the end of the sessions/day the drivers do practice starts on the grid, so there's some good opportunity for closeups.
Jerez on the other hand is a smaller track with less angles and interest through elevation changes tec, but the 'VIP' ticket allows you access to the main grandstand (uncovered, not so great for the monsoon of last year) and the inside of the track, which means being able to hang over the chainlink fence extremely close to the track in some parts.
Just be aware that any day that Alonso is driving (which will be two days of a four-day test) will be mental with crowds, the other days like a ghost town!
Dave