Most important, Lucene is an amazing search engine, and Compass provides it's features on top of Lucene (if Lucene was bad, Compass would have been bad by default, at least now Compass has a chance :-) ).
First and major benefit, since you already have a domain model, is that you don't have to work to map your domain model to Lucene data model (Document and field - think HashMap). You need to define the mapping and that's it.
The other major benefit is that you are abstracted from Lucene API's. If you tried to updated an entry in Lucene, you will see that you need to have an identifier, delete the entry, and add the new entry (all done using different index interfaces). Compass will do all that for you, and faster than if you were using Lucene core API's.
Other thing is that if your portal is updated dynamically, you can use Compass to update the search engine dynamically as well, thanks to it's transactional support and it's update support.
Last thing is, when working with Lucene, you have to write a substantial amount of code which if you were using Compass, you would have been abstracted from it (note that if you are batch indexing, Compass utilizes Lucene supreme indexing performance).
There are a lot of other things that compass gives you (common meta-data for one, a centralized place to define all your meta data, which sounds crucial in a portal type of an application), and much more in the future (the near one). If you wish to learn more, visit the Compass
site, and if you wish to continue to discuss it in more detail, you can use the mailing list at: Mailing Lists
A long answer for a quick question :-)