We sailed on the Queen Mary 2 from Southampton to New York in 2006 (photos here). It was a very worthwhile experience -- much different from and more satisfying than the regimented gluttony and tackiness of cruise ships.
One does not spend time outside on deck chairs in the North Atlantic, so there were many fascinating things to do indoors -- lectures by Oxford University professors and authors, acting workshops by members of the Royal Academy of Dramatic Arts, libraries, a planetarium, high-quality evening entertainment, and so on.
It's not the cheapest way to get across the Atlantic, but figuring six days of lodging and meals into the equation, it's not bad. And it was interesting to get a glimpse of what international travel was like just a few decades ago.
If you have the time, I recommend a crossing by ocean liner, at least one way. I'm glad we flew to England and sailed back, rather than the other way around. It was quite a sight sailing into New York Harbor before dawn, the top of the ship's mast passing only ten feet below the underside of the Verrazano Narrows Bridge, and sailing past the illuminated Statue of Liberty.