I'm a native speaker as well. As far as I know and often hear there is a little difference.
In my region (north and middle of Germany), it's normally used in a positive way. When you want something and it is reachable. Or if you want to explain plans. Another possibility is to use it to explain that you are something a bit more serious than other people.
- Ich habe die Ambition mein Abitur zu machen.
- Ich bin ein ambitionierter Geigenspieler.
For the first example Ehrgeiz is also common. The meaning is then absolutely positiv and more or less the same.
It is used with positive and negative meaning. Normally, it's a bit stronger than Ambition.
- Ich bin viel zu ehrgeizig. (Negative)
- Ich bin ein ehrgeiziger Student. (Both)
For (1) it means that you nearly do everything for your work (for example) and nearly nothing else. (2) can be positive or negative. Negative if you mean you are selfish in studying etc.