Happy "customer" here. I've been using their free 6in4 tunnel through OpenBSD for about five years and have had no mentionable problems. I configure mine solely with OpenBSD's network interface files, e.g. /etc/hostname.gif0:
tunnel <my current IPv4> <HE's IPv4 endpoint>
inet6 <my desired IPv6 address> 128 alias <HE's IPv6 gateway>
!route -n add -inet6 default <HE's IPv6 gateway>
I use the connectivity to reach a cluster of VPSes in AWS deliberately set-up without public IPv4 addressing, which would otherwise represent a large part of the monthly costs because of buttholes like Jeff Bezos actively monetizing IPv4 address space.