I hosted with them for about a year, they are a great company and are here in Asheville, NC! Nothing but the best of service and Steve is a really nice guy too. I would have stayed with them but I moved to AWS for some features that amazon has.

I haven't tried boost, but I use memcached, varnish and apc ... also use pressflow drupal for sites that need performance; mostly built to the spec fleshed out in this drupal group http://groups.drupal.org/pantheon it really does make drupal fly crazy fast! Cant wait to launch this real estate site, uses solr and a lot of other cool stuff; and of course an adaptive theme, need to update it to the latest 2.x beta though been holding off since it will require a few changes.