I am using an arduino Uno R2 with a version 5 grblShield. I had a lot of difficulty getting it to work, for a very simple reason-- the J11 jumper needed to be moved from IOinput to 5V. This was a hard problem because the issue is not mentioned on the main page or in the FAQ or in the Troubleshooting page or even on the schematics (should at least be a footnote). The only way to know that change happened from version 4 to version 5 is by going to the gShield 5 notes, which are in an obscure spot.
Specifically, the first answer to the FAQ:
https://github.com/synthetos/grblShield/wiki/grblShield-FAQ#arduinos
says that the gShield works with R2 and R3 versions and then the main page for version 5 says, "there is very little difference between the v4 and the v5. Both can be used on the Arduino Uno to run grbl, ...."
https://synthetos.myshopify.com/products/gshield-v5
Also, the schematics for the version 5 gShield don’t show the default connection on J11 and hence don’t show how Vcc gets to the board, this is reasonable in the diagram since the jumper is added after fabrication, but there certainly should be a footnote in the schematic about the need for a jumper there:
https://github.com/synthetos/grblShield/blob/master/hardware/gshield_v5_schematic/gshield_v5_schematic_pg1.pdf
This is one that should at least be added to the troubleshooting section, under “Motors won’t turn.”
https://github.com/synthetos/grblShield/wiki/Troubleshooting