There are two main types of hub construction. The first type utilizes a traditional ball-and-cone bearing system. Bearings, cones, and axles for these hubs are readily available at most bike shops. (Like most shimano hubs).
The second type uses pressed-in sealed bearings similar to those found in in-line skate and skateboard wheels. Sealed-bearing hubs are usually more expensive because they run smoother and have the potential to go farther between maintenance. (Like Phil's or Chris King)
The catch is, if you are using a sealed-bearing hub and it does have a problem (which I have seen happen with even the best), it might be difficult to find replacement parts in a timely fashion since most of these hubs require unique parts and bearing that are only available at specialty bearing stores or from the manufacturer. Therefore, for maximum reliability, I vote for the traditional ball-and-cone hubs made by Shimano. The XT models, if maintined properly, will outlast most rims. Although the traditional ball and cone hubs may require a bit more maintenance overall, they are easy to service, parts are readily available at even small bike shops, and they will not likely leave you stranded with a catastophic failure.
People often forget to mention that the freehub bodies are unique to the hubs. That is an extremely vital part of the hub. Although there are some advantages to some of the freehubs on custom hubs, none have freehubs that are as reliable and easy to repair than the Shimano freehubs...
Adventure Cycling Association