1) For defense, they are great against (normal) biters as they allow for a lot of firepower to be projected in concentrated chokepoints, but imho they're not really that useful as soon as spitters are spawned. Since they are not straight you end up with a non-straight line of turrets, which means that you often get in fights of 10 spitters vs a single overextended turret. This might be a bit better with laser/flame turrets, but I like my gun turrets . Because of their relatively small size, they often don't really offer good choke points for concentrated defense either.
2) For immersion, it would really help if it would be coupled to an actual height map or have a better generating algorithm. Now it really gives you a feeling of height in some places, but in others it just looks weird and jagged
3) In general, I really don't like the gen options. I tried very infrequent plus very large in the hopes of getting a couple large cliffs, but instead I get cliffs all over the place. If I do very infrequent plus medium I just get either no cliffs at all, or cliffs everywhere. On normal+medium I get no cliffs at all in a couple gens, might be the same as infrequent plus medium. Frankly, to me it seems like frequency is ignored, and size is a kind of frequency: very large generates cliffs more or less everywhere, while medium generates them either not at all, or patches where they are again everywhere.
Some examples:
Moreover, the cliffs in many cases just seem totally random (like a 'broken landscape'), or run parallel with water. The latter feels sort of natural, but is also mostly useless for gameplay.
What would I prefer?
a) it should be possible to actually vary size and frequency, and generate very infrequent but long cliffs, if needed with small gaps in between
b) in many cases, cliffs should follow terrain boundaries. This will make them perpendicular to the bodies of water more often, and give more immersion because of the terrain switch that goes along with the cliff.
For example, the cliffs I painted in below would IMHO be much more interesting, both from a defense/gameplay point of view and for immersion: