I had fun working with QGIS some years ago, connecting it to GeoServer, mapserver, importing shapefiles, and customizing a few maps. I didn't use as much as the GIS engineers I worked with, but it was definitely a great open source tool.

I had to use ArcGIS too, and while sometimes it performed well, when it didn't it was quite painful to have to deal with the local vendor to implement our features, and troubleshoot bugs in their software.