I've been really impressed with how easy it was to set up and instantly identify bottlenecks within our infrastructure. Sure, other plugins have offered similar statistics (and some with a nice UI too) but to provide all of this insight out-of-the-box and in a single tool is an amazing addition to Elasticsearch, and in my opinion will help boost even more widespread adoption.

If anyone is interested, I wrote about my initial reactions to Marvel here: http://chrissimpson.co.uk/elasticsearch-marvel-monitor-and-m...