Better than the rest, since the beginning
Not many people realize that Solano CI (formerly TDDium) has been around longer than most of the competition. So with that understanding, it's not really too surprising that they're so far ahead of everyone else.
Since I started using them (~3 years ago), they've had or added to the service:
* turnkey parallelism, out of the box with little or no setup required
* USEFUL parsed results (eg, sortable, filterable specs, collapsed to file name. You can filter to just show failed specs)
* capture of HTML body for failed capybara specs
* github integration
* sane defaults + project autodetection, with extensive configuration options (ruby version, firefox version, selenium version, etc)
* Caching of project dependencies + configurable cacheable artifacts (for ruby, this means your gem bundle & I setup a custom task to cache assets based on md5 of the assets dir so some builds can skip re-compiling assets)
* custom service support (have you gone and made yourself a SAAS mess with one app that depends on another? does your test suite depend on an api server to be running? this can be configured)
More features too of course but these are the ones I've found particularly useful and/or unique.