We just completed our first cruise on Seabourn -- a 14 day Norwegian fjords journey on the Quest which gave us a good mix of sightseeing and sea days. We were extremely happy with our first Seabourn experience, from the ship itself, the food & drink, the staff, and the destination.
For a point of reference, we've traveled on Regent for half a dozen cruises previously, and in the review I'll touch on a few points of comparison with Regent for others who might be considering either of these lines. We've always enjoyed Regent for the same reasons so many Seabourn cruisers love this line: the size of the ship, the quality of service, the quality of food and drink. It's been a few years since our last Regent cruise, but my feeling on our last Regent cruise was that the quality of food and service on Regent had slipped a little since our earliest experiences (back when it was Radisson). So we were eager to see how we liked Seabourn, and whether the nearly-uniform positive comments on the Cruise Critic Seabourn forum could measure up.
The answer: We enjoyed almost everything about Seabourn and the Quest. We thought the food was excellent throughout. The staff was exceptional: friendly, getting to know our names (even when we never identified ourselves) and our preferences. The ship seemed in perfect condition. We found the Quest very much on a par with the Regent Voyager and Mariner; we felt "at home" almost immediately, despite never having been on a Seabourn ship before. Of course, the Seabourn ships are smaller -- 450 passengers versus 700 on the Regent ships -- but the overall feel is the same. (That's a credit to Regent, that 50% more passengers doesn't feel more crowded.) The larger Regent ships definitely have a nicer theater, and they have a grand atrium that the Seabourn Quest lacks -- but those differences were pretty inconsequential in terms of enjoyment of the ship. (The pillars in the Quest's theater are a minor annoyance.)