How to evaluate hosted full text search solutions?

What are the options when it comes to SaaS/hosted full text search? How should I evaluate the different options available?

I'm looking for something that uses Lucene, solr, or sphinx on the backend, and provides a REST API for submitting documents to index, and running searches.

I could build my own EC2 AMI, but I'd have to configure EBS and other stuff, monitor it, etc.

Websolr provides a cloud-based Solr with a control panel. It's in private beta as of this writing, but you can get the service through Heroku.

Another hosted Solr service is PowCloud, also in private beta, which seems to offer strong Wordpress integration.

SolrHQ: another beta service providing a hosted Solr solution, with Joomla and Wordpress integrations.

Acquia Search offers Solr integration for Drupal sites.

If you decide to build your own EC2 instance, the SolrOnAmazonEC2 wiki page might be useful. Or you could just get LucidWorks Solr for EC2, which is probably the easiest and fastest way to get Solr on EC2.

Engine Yard provides a cloud-based Sphinx service.

Indextank is a hosted real-time full text search solution. It's pretty simple to set up (you can get an index running in a couple of minutes) and it's very powerfull (Reddit runs over IndexTank). It provides Java, Python, Ruby and Php clients as well as a Rest API specification. There's an awesome support service (including live chat). You should give it a try.