[kyletns]

I use Fastmail which has CardDAV syncing, and generally it's worked great. One or twice I've noticed it stops syncing and I have to remove + re-add the sync profile, but generally speaking I've been pretty happy with the results.

[troad]

I second Fastmail. The interface is very solid, and it syncs very well to my iPhone (I've not had any syncing issues myself).

They also have great (human) support, which I find important when it comes to backbone services like email, calendar, and contacts.

[palata]

Same but with Migadu. I handle the CardDAV from Thunderbird and from Android, it just works.