Germany is also among the most congested country in europe. It is a pain to drive there, as the germans are constantly rebuilding their roads, generating endless trafic jams. A8 karlsruhe- munich has never had a day without road works in 30 years.
I find british motorway better, the best being french motorway, but you have to pay quite a lot to drive them
Depends on where and when. On our road trip last year the Autobahn between Bremen and Köln was plain insane and probably the worst day of driving I ever had (my old car hadn't adaptive cruise control, maybe with the new one that'd be a little but better). The remainder, including the route Prague, Berlin and Hamburg, was easy going; even construction works were okay thanks to (regular) cruise control.
We did often drive on weekdays (but used public transportation in the three aforementioned cities P/B/H).
Traffic on the Autobahn follows quite extreme daily and weekly patterns. Pick the right hours and ypu can breeze through. Pick a wrong time and you'll have to contend with heavy traffic. Sometimes, the peak hours for traffic are a little bit odd. I have often seen the A81 between Ludwigsburg and Stuttgart packed on Sundays around 11pm, for example. The A8 between Stuttgart and Munich is regularly jammed on Friday afternoons.
Yes, it feels you are standing absolutely still when one of those mercs or porsches rocket past you (while I'm driving 130-140 km/h it still feels like standing still when some of them pass).
That doesn't mean one has to drive with full throttle, though :P it's just an option. I prefer simply starting the journey early enough and mostly go 130/140 (depending on the traffic) and make good use of recuperation when speed limits start or on steep downhill segments. That's way more relaxing than going 200 or more (which I could, uphill).
Germany has one of the better/higher quality roads and highest speed limits in Europe, though.