They target their offer a bit differently than other providers and this puts off some people, who don't appreciate the tradeoffs.
The details and tradeoffs are a bit too many to list here, but basically when they can cut corners to lower prices, while still keeping server-grade performance and uptime, they do not hesitate to do it.
It's also a bare metal / every server for itself / full DIY offering, without expensive gear such as SAN, blade servers and such. So if you're not prepared to roll your own virtualization, failover, backup, provisioning, etc solutions, you are better off going somewhere else.
That being said, the network service and server specs you get per your buck are second to none. The support is also adequate, if you understand the limitations of their offering.
Source: been a customer for 10+ years, currently have 10 servers with LXC and ZFS-on-Linux, hosting 100s of websites, mail domains, and custom applications, including a full backup solution based on ZFS send / recv. Would cost me a fortune on any other provider.
They'll answer competently. I sent a spam complain there once and got a sensible reply from the CEO.
But if you buy a product where they supply hardware and you do all the root work yourself, and then ask them to help you recover after a file system problem, they'll brush you off and you might consider their response rude.
Funnily I saw a thread on the customers forum at forum.hetzner.de today about why Hetzner does not have a better reputation social-media wise. Seems not to be on their priority list. There is even an English subforum, you may try to register there and ask for experience by other customers.
There is also the 'German' view of customer service to consider (i.e. the customer is not always right).
Caveat: (Root servers) They won't help you find problems or solve problems. They don't monitor your hardware. If you've got hacked and send spam, they close you down. If you have harddrive problems, you need to show them otherwise they will not easily swap drives. It's bare bone, though if they are responsible for the problems, their support is fast when notified. They are also helpful, e.g. when moving servers to a different data center, keep old servers so you can migrate to new ones etc.
Edit: be aware this is a bare metal server. You have to do everything yourself.(monitoring, raid integrity, smartd). And do not forget to request a remote KVM console(LARA), if you plan to reboot and you are not sure the kernel will boot.