I've been using Ableton since it came out and I too use it as a songwriting tool. Actually, for most of the demo's I do, I track and mix in ABLETON. I love the audio to MIDI functions and most of the "in the box" audio effects. I used to use Ableton through rewire to Protools but I'd say now I use both 50/50.
As a drummer primarily, it's great to set up a heap of ideas and trigger off the clips using my Roland TD-30 and record the results.
I love hearing from others who use Ableton, I'm happy to share info and tips too.
Whats your favourite feature in Ableton and have you ever done a full mix using only Ableton?