We just did our first cruise to Europe in May and it was the most incredible trip we have ever taken. It was a twelve day Barcelona to Venice on Celebrity's Millennium. Visited Barcelona, Villefranche on the French Riviera, Livorno (Florence, Pisa, Tuscany), Civitavecchia (Rome), Naples (Pompeii and the Amalfi coast), Santorini, Athens, Dubrovnik and Venice. We are diehard Celebrity fans, but like Princess too. All depends on what countries sound most attractive to you but we were in heaven on this cruise. We went in early May because the weather is not so hot and the crowds not as great as in summer.
Actually have booked the Star Princess for October 2004 doing almost the same itinerary from Venice (minus Greece and Dubrovnik) and with a transatlantic tucked in at the end. The only thing we could complain about with the itinerary we did is that it was so packed with ports that it was very intense- not enough time for relaxation. So next years will be perfect... have the transatlantic at the end to kick back after all the running around in European ports.
Rates have just been released on Celebrities sailings and the May Millennium and Galaxy prices look nice. Just one thing...don't go to Europe without seeing Venice. Most magical place in the world.