I've used their cheapest monthly VPS for about 18 months. Provisioning was excellent, uptime also. On the one occasion I needed support they were responsive and my problem was solved promptly. Web-based console access a nice feature.
They experienced a break-in a few months back. Communication to customers was clear, timely and reassuring. (All companies experience problems at some point, it's how they react to those problems that is important. Hetzner's response was very good).
Some small amount of their site/documentation is in German. That's not a big deal (google translate or whatever) and generally it is only the more obscure corners.
Disk size might be limiting on the cheaper offerings (20GB / 7.90 EUR).
I have not experienced the "heavy firewalling" mentioned elsewhere in this thread but I haven't tried putting any serious traffic through it.