Since I've only been paying for domain registration while using email for free, I feel that I can't complain as much, at least not in good faith.
Still, both the freebies and the paying customers get 6 months to decide which is all I can really ask for. GandiMail has plenty of room for improvement so as long as they can justify the cost increase, I'll consider staying.
I think we as customers have to manage expectations and at least let the vendor process their changes before making our decisions. That's what the 6-month period is for. If thousands of customers take flight immediately after the announcement, the rest of us might end up with a lesser service than we otherwise would've. It has to be subsidized somehow, clearly domain registration fees weren't doing the job.
The free email is the only thing that kept me with them.