This year at Oktoberfest I am using an O2 Prepaid-Surfstick and by far it's been the biggest pain in the ass. Using the O2 network this year at Oktoberfest and my apartment (just a few hundred meters from the wiesn) has been the worst. Other friends and colleagues in the area have also experienced O2 issues too. I've easily had better Internet connections from other telcos in worse places. Hell, the connectivity at the Chernobyl nuclear disaster site was better.
The HTTP transfer speeds have been incredibly slow, but there's also been incredible latency and very frequent outages; it's happening dozens of times per day with this O2 Internet stick. This is supposedly on a 3.5G+ network while the transfer speeds are commonly just a few Kbits per second.
Setting up the O2 Internet USB-based stick also required three trips -- and buying two separate one-month pre-paid plans -- to the O2 shop near Marienplatz to sort out various issues. I'm also doing this under Mac OS X and not Linux, since O2 nor ZWE (the manufacturer) provide official Linux support or an O2-catered client.
It's been awful for a week and a half, but thankfully there's only one more week to go. To all the other Phoronix tech readers going to Oktoberfest or Germany: you're likely best looking elsewhere for mobile Internet connectivity in Germany. I'll have to add it to my Oktoberfest guide for next year.