Barcelona is such a comfortable city, with so many good hotels, reliable taxis and excellent service personnel that I don't see why it would be necessary to pay the premium prices for a hotel arranged by Oceania. We booked the Hilton for free on points, but there are lots of options and it would be tough to go very wrong.
We were on the 2007 Regatta crossing from Barcelona to Miami as well, and it was a great cruise. There was a 30-hour stretch of pretty wicked weather--by far the worst we've experienced in four crossings--but what can you do? It's the ocean, after all, and a relatively small ship. Fortunately, we don't get seasick. Lots of busted crockery and furnishings, though.
In addition to the good things Howard White had to say should be added the overall ambience created by our fellow passengers. Great bunch of folks to cross with.