...well, I make the basic terrian with Nem's Terrain generator.
http://countermap.counter-strike.net/Nemesis/
It seems to be a bit more comprehensive than Terragen, I love the "lock" feature which allows each vertice to be locked into place once you get it where you want.
I'd still say though, that about 80% of the work is pretty much brush by brush (or small group) manipulation. After a few hit or miss attempts it gets pretty easy to get what you want/need.
I am playing around now with an entire map done entirely in rock/terrain brushwork. This one is requiring some tedious manipulations but may be worth it in the end. Here is a pic of one small corner with a simple -minlighta compile...
http://www.backshooters.com/maric/cave.gif
I am hoping that the complex terrian will be detailed enough without becoming too tiresome. It is literally four seperate
.map exports, all from using Nem's. Q2 is choking a bit and giving me the occasional Vismap_expansion Overflow but there is always the Func_wall "cop-out" if needs be.