The Underdark is often described as an endless maze of twisting tunnels, which makes it a little intimidating when it comes to mapping. I've toyed with the idea of trying to combine maps that others have already created... I'm a big fan of the Lower Broken Lands from the Mystara setting and their corresponding maps. I've also considered hex maps, as well as skipping the mapping altogether and using a more abstract method. In the end, I get tired of these ideas really quickly. They're either too abstract for my tastes, too much of a rip-off of someone else's work, or far too time consuming to bother with. Geomorphs might be the perfect solution, though.
If you increase the scale so that tunnels are much larger (which, in my opinion, often makes for more interesting combats that tight quarters), you could use any of these geomorphs to plot out large sections of cave systems quickly, so at least the major tunnels will be ready to go. Then you can add the smaller side passages as needed.