The documentation set I work on uses olinks for just about all of the cross references. Their flexibility and the fact that they are not resolved until publication time make them powerful and ideal for modular documents.
Like xrefs they can use the DocBook gentext system to build the text for a link and like link they allow you to provide your own link text. Unlike either of them, they do not use ids that must be resolvable at validation time. In fact an unresolvable olink is not a validation error because they can be used to link to documents outside the scope of the current document such as another book in a product library or an article. The resolution of the targets to actual endpoints is left up to the publication system.
This flexibility, naturally, is both friend and foe. On the friend side, it makes writing modular documents easier by eliminating the validation a errors that crop up when trying to create a link between two XML documents that will ultimately end up as part of the same publication. This is one of the main reasons we started using olinks instead of xrefs. All of the writers found the constant validation errors distracting. Naturally this is only a problem if you are writing DocBook XML documents with a real XML editor. Teams that use text editors with some syntax highlighting and auto tag closing features will not be distracted by the validation errors. Of course, they also won't know a document is invalid until it blows up in publication.
The other strength of using olinks is that a team can architect a library that is more that just a loose collection of related books. The books in a library can have working links between each other. One book can reference a topic that is discussed in another book and provide the user with a functional link directly to the required information. This is possible without olinks, and I have worked on libraries that attempted it. However, without olinks, or some similar mechanism, deep links between books in a library is a bear to maintain and most resource constrained teams will not succeed. The argument can also be made that deep links between books in a library is not valuable. Given the difficulty of maintaining them using "standard" methods, the argument is correct. However, using olinks lowers the cost to the point that not using them is letting your readers down.
On the foe side of the equation, using olinks does add some overhead to your publication system and adds some extra steps in setting up a library. If you are using a publication system based on the DocBook XSLT style sheets, the added overhead is fairly minimal. Most of the work is already included in the base style sheets. You simply need to turn it on and add a preprocessing run to the system. The preprocessing run will calculate the target databases needed to resolve the targets to real endpoints. We currently have our system set up so that an author can skip this step when doing builds of a single book for test purposes. However, the preprocessing is done by default when building an individual book. When building the entire library, all of the books in the library are preprocessed before any output generation happens and that step cannot be skipped.
The added steps in building a library are not all exactly steps. Some of them are things that all of the writers working on the library must keep in mind and rules to which they must adhere. The first added step is determining the layout of the generated HTML site. This allows the system to calculate the relative paths between all of the books in the library. Part of the site mapping process includes creating book IDs for each book in the library. These IDs are used to identify the link targets when they are outside the scope of the current book.
Most of the remaining overhead involves sticking to a few simple rules. You shouldn't change any IDs once they are in use without making sure you change them throughout the entire set. You should rebuild the target databases whenever you make changes to a book. If you do make changes to IDs, rebuild all of the books in the library to make sure the link targets resolve. The most important thing is to watch for warning during book generation to ensure that all of the links resolve. Mostly, these are all just basic best practices is any writing job.
So there are lots of benefits for the minor costs. A responsible, well disciplined team of writers should be capable of using olinks without a problem if they have the need for linking between books or are doing modular documentation.