Gandi have fantastic service and don't try to upsell you unlike some other registrars. Note that they are quite expensive for some TLDs (like .com and .net) while relatively cheap for others (like .io and .cx) so I'd really only suggest using them for TLDs where they have a competitive price.
Working with gandi has been a breeze. After starting and transferring various .IO domains there, I've found that their site is easy to use, modern, offers features some sites don't (DNS for example, where places like nic.io do not) and the changes have been made promptly to the domains.
In addition to just working(tm), the site is intuitive and easy to use.
Slightly on the expensive side if I remember correctly, and the username system is somewhat annoying. But everything else is perfect, you have perfect control over your DNS settings and you can even use the same settings on several domains so that management over several domains is simple. As they say themselves: "no bullshit."
I have bought 3-4 domains from Gandi
I've been with Gandi for about 4 years now and I've never had problems. Their support its great, even when I bricked my phone and needed my 2fa reset they called asap to reset the codes.
I've had nothing but the best experience with Gandi.net for registering .IO domains. Their user interface is clean and their approach to the domain registrar experience is exactly as they say "no bullshit".