We have been using the Leica Geosystems GG04 plus for quite some time with great success. We mainly utilize it with with an iPad using the Esri Collector for ArcGIS app for high-accuracy GNSS positioning (1-3 cm is typical). I have been in some pretty challenging environments with the GG04 plus and other receivers wouldn't provide the level of accuracy the GG04 plus offers. We also subscribe to their SmartNet correction service as well and able to utilize GPS, Glonass, Galileo and BeiDou in our area. While Galileo and BeiDou are not fully operational yet, they will provide more benefit in the future.
I would strong recommend taking a look at the GG04 plus as it has been a work horse for us.