I don't have to worry about scaling my ELK stack, those considerations are handled for us. Additionally the support is terrific, very responsive and the API documentation is outstanding.
Several times we have used logz.io to track down issues in real time with ease. In addition, if you have a suggestion or question, the response time is outstanding.
Easy to use, intuitive tool helps us and is very useful in what we propose to do. We're reasonably satisfied.
Subscription plans should be more progressive, sometimes if the maximum event ingestion is met, the events are lost.
Ad hoc parsing of messages and extraction of values (a la Loggly) is missing. Missing ability to set all filters in GUI before the query is actually performed (suspended update).
Elasticsearch complexity requires some hand-holding, i.e. index failures. Some slowness in searches when the service is busy.