I work for Swiftype, which provides a cloud search plugin for WordPress.

We definitely see the advantage of off-loading search to another server. Plugins that use your local database like Relevanssi can't handle lots of posts, whereas we have customers with hundreds of thousands of posts happily using our service. Additionally, we give you nice search analytics and the ability to change result rankings by dragging and dropping.

If you're not on VIP (or if you are -- our plugin is VIP-approved), give Swiftype a look. It's free for small sites.