What do you like best?
It lightness and vast diagrams support. Also, it's open source and plugin able.
Besides it's portability, it has a Linux, MacOSX and Windows versions, it made Dia a swiss-army knife like tool, you can quickly draw anything you need, from UML diagrams to Electrical Engineer like diagrams, going through Cisco diagrams and much more.
What do you dislike?
It's exporters, aren't very intuitive and some of them fail on rendering complex diagram with awkward results.
Also, at least on my not-so-old laptop, with Ubuntu 14.04, the graphics has some glitches when drawing, it's really annoying when you'r trying to fix little details.
Recommendations to others considering the product
Allow team work collaboration/versioning in some way, if it would format to a XML or something like that, could be simple versioned with a any CVS.
An integration with Cloud solution could help too, like OwnCloud or DropBox.
What business problems are you solving with the product? What benefits have you realized?
Diagramming and designing class diagrams, it deployments diagrams, use cases, BPMNs, flow charts, and much more.