I've used Grass on it's own in the past, however I really enjoy the fact that it is available within QGIS within a plugin. This makes using the best features of GRASS easily accessible.

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.

I like the scriptability it provides to users in a Linux environment. At the National Weather Service, we used GRASS to map hydrologic, soil, and geologic information to create flash flood risk maps. The connection from GRASS to bash/korn to the R language was smooth and light weight. The open source nature of this program also provides huge opportunities to not only contribute, but reap the benefits of other scientists contributions. Specific to hydrology, a number of people have created some great tools.