I’ve had occasion to need it the past few weeks. I’ve tried drum parts, also acoustic guitar (both picking and chord strumming) and even bass.
I usually write in 4/4. Typically what happens is that I get a ton of fine grained tempo changes but the time signature set to 1/4. I then often need to divide or multiply, although if memory serves I can only divide by 2 once and then the option is grayed out. More often than not, I don’t end up with a usable tempo track.
I think the concept is very cool, and I’d love to try some free form acoustic songs and map the tempo to them so I can let the feel of the song dictate rather than be a slave to the click track (or have to write out tempo changes beforehand). I was wondering if others have had better luck, and what suggestions you have that might improve my results.