Mobile data in Germany? (but D1/D2 only)

[inquisitor]

Originally Posted by fuxoft

Again, the call-rates are non-issue because I'll really only be using Internet.

I understood that, but I mentioned those call rates as an extra advantage beyond the cheaper data tariff.

The more important question is: What is the signal quality of Vodafone vs. Fonic? I'll spend most of those 3 days on the highway or in smaller cities. I don't need FAST internet, I need RELIABLE internet.

O2 also cover nearly 100% of the country and they have EDGE (up to 200 KBit/s) on their whole network, so you should always get a signal and reliable data connections, but Vodafone of course provide better 3G coverage as you can see from this map :

Also, I need to use the card as soon as I cross the border from the Czech Republic and buy it in the first larger German city (Dresden, probably). How fast is "switching to Tagesflat"? Don't I have to wait several hours for it to take effect?

No problem. All Fonic retailers are listed in my earlier posting. E.g. Fonic is sold at Lidl supermarkets and it can be activated immediately by phone, too.
The switch to the "Tagesflat" is carried out while you are still on the phone with customer care. It's about seconds and you will be notified of the switch by SMS.