That said, you'll need a dedicated server just for search or you'll have to farm it out ($350/year for a 1000-2000 page site using Acquia search for example).
Other than that, the only one I've used a lot is Drupal search. Generally, I would say it is not as good as a custom Google search engine for your site. It gives different results, but I would say generally a bit worse. That's been my experience with most built-in search, but I can't say I've had a ton of experience.
Thanks for the reply ergophobe. Budget and server access is not an issue. I'm working for a large company with an open source ethos, but that will farm stuff out if necessary. I have a lot of resources available. Just want to make sure I'm launching into the right solution.