O2. 28€ / 5 GB + 10€ "Datensnack" (exceed). The 500 MB thing is a commercial I saw a few days ago "LTE Einsteigertariif" with a flat rate telephone. Actually not that bad, but 500MB LTE...

I have a feeling I'll switch soon. I just took O2 because I had a good experience with their prepaid rates, and having checked T-Mobile and Vodafone I didn't find it better.

I've just signed up to WinSIM who are O2. They have some good cheap packages if you are a low user of SMS and calls but do use data. I'm paying €4,99 per month for 1GB. Calls are €0,15 per minute. I've just started so I'm not sure what the catch is yet!