Ranked in these QuestionsQuestion Ranking
Supports REST, WebSockets and MQ protocols.
Pro Open Source
Kuzzle is open source and it's released under the Apache 2 License.
Pro Realtime through filters
You do not connect to classic rooms but to search filters and can be warned of changes on permanent storage or volatile one.
Con May have issues since it's stil la relatively young project
Kuzzle is (as of February 2nd, 2015) in alpha version and approaching beta.
Con No hosted version
Kuzzle has no hosted version. In order to use it, you have to install it to a remote machine. This can be cost-effective but it also adds overhead because you will have to take care of the hosting and backend infrastructure yourself.