Based on my experience with wearing out a couple Ultegra Hubs I have found them to roll smooth when new expect they wear out fairly quickly. When I say quickly, I mean noticeable difference with in 5,000 km. At 15,000 km I figure there is a 5 to 10 % energy loss. My feeling is my wet weather riding style does not vibe well with the hubs. Their weak point is the hub casing. Grease, bearings and holders can all be replaced but the casing still wears out.
Additionally, the cost of a replacement free wheel is close to the cost of the hub. Overall good if you are using them for less then 5,000 km / year and keep up on the maintenance. The would last two to three years.