I had a map where the lava pipe actually extended up into air under its own support with only one side touching a cliff face for 25 Z-levels. I cut a slot 2 wide in the far side (from the cliff) of the pipe with channeling.
The magma fell 30 Z-levels into a pit I had dug for it at the base of the pipe, flowed through a set of underground channels and then flowed into the top of a canyon containing the brook I had there (AFTER it went over its waterfall), thereby creating a taller and taller obsidian dam.
A dam which I used for some really nice water bomb traps. And mined obsidian from. And I even got to tap into the channel later on to power my magma forges only 5 Z-levels under the surface.
Unfortunately, later on that dam reached up 28 Z-levels to the channel outlet and then dammed the lava outlet, and it ended up flooding my fortress with water from one end and magma from the other.
However, one thing to remember:
If you're playing with magma, always have a way for it to drain away (and be very very liberal with your drain sizes), or have a way to counter-flood your fortress with water. That last one may sound silly, but it's a hell of a lot easier to recover from a water flood than a magma flood, and the water will flood faster.
One of my most memorable experiances from this game is watching a dwarf run down a corridor away from a wall of water and running straight into a magma flood, and just standing in between them as they both advanced on him. He ended up getting encased in obsidian.