I've done Barcelona, San Remo and Monza (twice) (and Silverstone but that's not the continent!).
Barcelona was the most modern with good facilities. It was a corporate thing so we had the perfect seats.
This is the Video of the trip. /ubbthreads/images/graemlins/cool.gif
San Remo was also really good. We did it as part of a Euro driving tour. We bought some good but expensive tickets. You miss out on a lot if you're not in the pit straight.
Monza - I did as a corporate day and a private visit. The corporate one we were stuck in an open Grandstand (no shelter from the scorching heat) on the back straight so they just flew past us. Not the best position. When I went as part of another Euro tour we bought general access tickets on the day from the touts. I even got them cheaper than face value despite them starting at a silly price! Watch out for fakes as they are all scanned on the way in so they spot duplicates etc.
Monza is a scummy GP to see if you're on the inside in the forest. People camp there for days and it's all a bit unhygenic but it is the best atmosphere in the world (if you're wearing red!!) and worth a visit at least once.
If I was doing one this year I'd choose one I haven't already done. You can do Monte Carlo reasonably cheaply if you choose a hotel with a good view. This was ours and we was there the week before the GP (just after we'd been to San Remo). The rooms weren't that expensive (something like £190 a night).
I have more videos but have yet to edit and upload them. /ubbthreads/images/graemlins/crazy.gif