Vodafone and T-Mobile have in my opinion, although the better mobile network, but fall for different points by either because they Zwangsproxy are contaminated or tariffs too expensive or there are no suitable data packets.
The o2 free ticket offers as many advantages. Clear the prices of 15 cents per SMS / minute are not really up to date. But: For charging there's extra units. Although you get the initial bonus SMS tariff unlocked, but you easily switch to the other two bonus options (goes for free on the free O2 hotline). I immediately after the activation of the bonus SMS / free minutes Option changed. There's for 15 Euro charge 50 SMS / minute to all networks mind. For larger amounts there are of course more bonus.
You can also submit the usual (Internet) also book options.
I booked the internet packet M with smartphone option (15 euros per month) 1 gigabyte of data is included, and then is throttled on GPRS speed. I charge for each month 15 Euro "must" visit for 50 SMS / minute on top. That's pretty low as I find.
If you can live with the O2 network, for that is here definitely worth a look.