What do you like best?
The system is very robust and with version 8 has removed much of the arcane manual tuning that was needed in older versions. If you are storing and manipulating XML documents, it is very simple for the amount of power it gives you. MarkLogic has excellent documentation and support ecosystem. There are usually other people who have encountered similar issues in the past and their development mailing list is active. Their recent integration with REST, JSON and node.js has substantially simplified building integrated systems on top of MarkLogic. With the recent additions of Active Directory and JSON as first-class-object, it is possible to build a system from the ground up very quickly using only MarkLogic and world-class front-end tools like AngularJS or EmberJS. MarkLogic's support for fully ACID transactions is very rare in the NoSQL market and their reliability is significantly better than anything I've encountered in the open-source NoSQL ecosystem.
What do you dislike?
MarkLogic has moved somewhat away from the pure XML world and has not kept up to date with XQuery 3.0, XProc, XQuery Update Facility, etc. They seem to prefer to enhance their proprietary features rather than implement additional standards compliance into their product. I would like to see them improve some of their basic functionality like normalizing their API and refactoring some of the more confusing areas (HTTP FORM handling, native file read/write/delete, etc.).
Recommendations to others considering the product
MarkLogic is best utilized by a team that is knowledgeable in XML / XQuery technologies. Despite their inroads in JSON and REST, the learning curve for people without significant XML experience is quite steep. However, if you have invested in XML / XPath / XQuery / XSLT knowledge, then you can be up and running in MarkLogic very quickly. MarkLogic is a great alternative to non ACID compliant distributed databases like DynamoDB / Cassandra / MongoDB. The enterprise version is pricey, although for personal projects, education and non-profit uses they do offer substantial discounts and the community edition still supports the important NoSQL and ACID features.
What business problems are you solving with the product? What benefits have you realized?
I have used MarkLogic as a Document Database in a NoSQL environment to support Fortune 500 customers. We have build custom web-content-managment, component-content-management, document archival and publishing. We have also used MarkLogic as a centralized metadata repository to integrate with governmental databases for purposes of normalizing data governance across a large civilian federal agency.