FYI: I just decided to not to extend my contract at o2 Germany.
Reasons:
1. Often poor UMTS/3G signal - you'll see people with Vodafone getting about as twice as much downloading speed as you do

2. 200mb data volume - I usually hit that mark on the ~20th of each billing period (would be much earlier if I didn't try to avoid traffic all the time, e.g. asking a friend if he could quickly look sth. up on google so I would not have to do it).

3. I was able to get a much better price at Vodafone:
For 29,95€ per month they offered me:
Unlimited calls to Vodafone + German landline
Unlimited texts to Vodafone
Unlimited calls (for 12 months) to a german provider of my choice
UMTS Flatrate 5gb (after hitting 5gb -> GPRS)
40 texts to any provider

+ They do not care if you use skype and/or tethering with your mobile phone. Friend of mine uses skype all day and didn't receive anything asking him for some extra money due to the usage of voip.

I hope my post was helpful.