I like how easy it was to set up, use with my existing orgs without having any additional training/support costs (it's been very well documented).
The support team can be difficult at times but they are working hard towards making things easier! Some of our users have reported issues in some countries where there isn't always 24hr access so this could cause problems if you're looking into using GP as your main mail platform only - make sure everything meets compliance requirements before proceeding otherwise risk getting blocked by regulators due to lack of security protocols etc.