What insurance should I get for a car rental in Germany and has anyone ever used Sixt?
Sixt is okay, but they don't always have the cheapo if that's what you want. I suppose the free upgrade to something bigger is okay, but I don't like it since it ups the gas price and I tend to cover thousands of miles. Your credit card will cover the insurance. I use the AmExp deal for twenty bucks a rental period. It gives you a tad better coverage than Visa/MC and it has something like a forty-day limit while the other two are something closer to thirty days.
Don't know what airport you're renting out of but this is an interesting thread on one company at FRA - http://www.tripadvisor.ca/ShowTopic-g187337-i301-k2464569-o100-Don_t_rent_a_car_from_Thrifty_or_Dollar_in_Frankfurt-Frankfurt_Hesse.html
Sixt and Europcar are the two largest rental networks in Germany, market leaders. You'll be fine renting with either one of those.
Don't forget to check prices using Kayak, and go to the Autoeurope.com website, where you cna cehck pricing based on rental agency, pick-up location, etc. As stated, your VISA or AMX card should cover the collision/theft deductable that the rental agency only provides for "rip off" amounts. Read your contract closely!