W2GO, Tennessee:
two verticals, a Cushcraft R-8 and now the larger Steppir about 5 feet from the beginning of a downward heavily wooded slope. The slope is at least in the 20 degree range and in parts steeper. It goes down several hundred feet to the valley below. Both antennas are absolute killers- I hear 99.9% (low noise floor here) of what's out there and a large portion of that is in the log.

K3WA, West Virginia:
On the side of a small mountain.  At one end of my property is a sheer drop off of about 70 meters at about a 70 degree down angle towards the North/Northeast.  The other direction is a sharp rise at about 50 degrees up angle of about 120 meters towards the South/Southwest. Last year I had both a multiband dipole and a vertical installed very close to one another.  The dipole always outperformed the vertical to the North/Northeast (downslope).  The vertical always outperformed the dipole towards the South/Southwest (upslope). 

WW5AA, Arkansas
As with any ground plane, whether real earth or radials, my experiments indicate that distortion and signal loss in the reflected ¼ wave only begins at angles greater than 45 degrees.

DJ6QT about CT3-experience:
With steeper terrain verticals seem to loose a small bit about 2-3db for a 3000 km distance

DF3KV 
comparing an inverted V with feedpoint at 105 ft (35m) with a quarter wave vertical  for 80m both close to a 3-7 degree downslope - with no difference between both antennas monitored by a lot of DX-stations

DK1NO:
Comparing on 40 m an Extended double zepp (EDZ) with an vertical array of 4/8 elements - with the vertical gaining cmparatively more from a steep downslope than the EDZ

N5TW:
About a 4-Square-inquiry: Modeling showed that degradation  would be slight but it is always good to find real world evidence.... I received several helpful replies including two from folks with good  experiences with bigger slopes than mine... Pleased to report that my new 160M Inverted-L based Comtek 3-Square is alive, kicking and 
performing well over a 5% terrain. 

VE1JF:
My 80M 4sq faces everything from uphill slopes to a clear shot over salt water to the horizon from close to 200' altitude.  To tell you the truth, I don't notice that much difference.

W7WHY:
I live on the N.E. slope of a hill that is about 450 feet ASL. The hill slopes off at an angle of about 30 degrees towards Europe. My experiences with a 3 element vertical array is that it kicks butt in the direction of the slope.  I constantly beat out guys here in town working dx.

LZ2CJ (in 2000):
At LZ9A we have an Inv.Vee up 27m at its apex with ends at 10m. There is also a based loaded 27m high vertical
with 10 ( 40m long) elevated radials. Both antennas are on a 700m high hill with acceptable ground conductivity ( not only rocks). Inv.Vee is never better, than a vertical as transmitting antenna, (sometimes difference is small ,but always exsists).

I4JMY:
Meanwhile an horizontal antenna benefits up to 6db of real gain at some elevation angles (and nulls at others to keep E=mc^2) independly by ground constants, a vertically polarized quartewave antenna hasn't any other chance than a maximum theoretical gain of 3 dB, at low elevation angles and if ground is perfect. ... A vertical placed on the top of a sharp hill is often more comparable to a largely elevated monopole with his artificial groundplane than to a vertical placed over a sloping ground.

KM1H about 160m-experience on hilltop-QTH:
Later the 160' tower went up and the inverted V went to the top. ( The granite was only 8 to 14" down at the tower base.)  For the next 4 years I had no real problem working DX but I realized it did not have the punch of the old vertical. Finally I decided to try a vertical with elevated radials (wire hanging from a top guy wire and about 25' from the tower; the bottom sloped out to 65' from the tower). The results exceeded my expectations. At no point was any DX better on the horizontal; most times there were S units of difference in favor of
the vertical. When possible I ran numerous A:B tests with DX...all hours, all paths.