What do you like best?
The best thing with using Grass GIS is the functionality that it offers. As reported on their site, Grass GIS is used for geospatial data management and analysis, image processing, graphics and maps production, spatial modeling, and visualization.
There are a ton of features that can provide solution to most requests a user can have, and are not included by default in other software (at least not for free). From Remote Sensing and Satellite/LiDAR Imagery management to Hydrology/Groundwater/Wildfire modelling, Grass is able to tackle most challenges that it comes up with.
Another great thing is that Grass is, nowadays, a component integrated within QGis, so someone can benefit from all of the Grass tools within QGis, without having to jump from one software to another.
What do you dislike?
The UI/UX is a bit difficult to use for someone who is accustomed in using ArcMap and QGis, as it is a bit more complex.
Additionally, the data management follows a different logic in contrast to ArcMap and QGis: data cannot simply be "added" but imported, which means that they are stored in another (pre-selected by the user) folder, and then exported back when done with the Grass work. This can be a bit time-consuming for a user.
Additionally, Grass GIS covers a broad set of tools, but unfortunately each tool is not going into much depth - there are useful features missing from tools, like Origin ID and Destination ID in network cost isolines.
Recommendations to others considering the product
Grass GIS is one of the most valuable, free and open source GIS software you can find out there. You can solve very complex problems, without having to pay anything - if you enjoy though, donations should be more than welcome :)
It is certainly a software that every GIS user should have in their toolbox.
What business problems are you solving with the product? What benefits have you realized?
The business problems I am solving are around spatial data management (creation, editing), geo-processing data management (buffers, intersections etc.), and spatial analysis (network analysis etc.). The problems I am solving using Grass GIS are mostly around advanced spatial analysis, that cannot be solved using other GIS software.