I'm an amateur/hobbyist songwriter and producer. I also play keyboards and guitars live with a band. I've found Ableton Live 8 to be a really valuable practice, writing and performance tool.
For recording, I use Live like a sketchpad. The "clip" concept lets you easily loop certain phrases or samples (like a guitar part or drum loop) and figure out other parts without having to keep stopping and restarting playback. Also, being able to trigger individual clips dj style really lets you be creative with how you're going to arrange a song.
I was really impressed with the "looper" effect, which is basically like a looping pedal in live. It's been a lot of fun experimenting with layered guitar parts, etc. I've also found the warping function to be great. This lets you basically stretch something you recorded to be slower or faster, without any change in time. It also lets you clean up your recorded parts if you were a little out of time.
The other thing I really like about this software is that it's very stable, even using other companies plugins. I use a lot of VSTs, and I've never had a problem with live.
Overall, a really great product for studio or performance. It's easy enough to pick up that you can start having fun quickly, but deep enough to really let you go as far as you want to go with programming, mixing, etc. Two thumbs up.