Eloqua is very powerful and very customizable. It goes beyond the traditional categories of email marketing, lead scoring, &etc into true marketing automation (Eloqua likes to say Revenue Performance Management, which I'm not sure we're entirely at, but the promise is really there). The out of the box reporting is great, and integration with Salesforce.com in particular is solid. The solution is also extensible, and has a lot of "Cloud Connectors" which allow you to integrate external services without writing any code. Eloqua also has EXCELLENT customer support, whether that's through their training programs, their Customer Success department (shoutout to Adrian Chang, who is really great), or frontline support.
Room for Improvement:
The UI of the reporting tool (Insight) is a bit dated. The out-of-the-box reporting supports only one top-level funnel, so it's great for reporting to the CMO, but less so when you have eCommerce, Channel, and Direct all with different funnels, for example - this requires custom reporting.
Broadly, there are many services which provide simple email marketing. Go with Eloqua when you want to move beyond that simple paradigm to true marketing automation.