The BC coilovers with my selected rates isn't harsh at all. 
Stiff enough for spirited driving, soft enough for every day roads. 
Those who have complained before is running BC default rates. Good for track, not so great for your back.
It is all about choosing the right rates for your application.
 

MR2QIK

I have King springs on mine, they haven't sagged after 10years. Ride quality is not pleasant though, but I managed daily driving for many years. I had them with adjustable shocks once & the ride was much better. The springs themselves are ok, just not well mated to the original shocks.

Having said that, I'm sure I'll be blown away if/when I eventually get BC coilovers.

Kings give a great ride height. I honestly expect they'd be good with Koni's.

that depends, are the spring rates matched to the koni's?
Do the kings compress at the same rate when both springs are rated the same?
 

MR2QIK

Kings give a great ride height. I honestly expect they'd be good with Koni's.

adisco

I prefer any springs over 90% of coilovers.
Most coilover are way too harsh for my liking.
In my skylines i ended up rippind out coilovers (teins) ans going for kings.
My old s15 came with coilover, ripped em out in favour of lovells springs, never looked back.
And now im on kings in mr2.
Hate coilovers with passion and unless i was driving on perfect roads 100% of the time, id never get them.
(Going against the tide? O well my opinion)

I concur  with this^
I've had Tein HRs in my car for a while they were stiff as **** like most Jap coilovers! just as well the 55 profile tyres absorbed some of Adelaides poor roads...I don't hate coilovers but they're designed for track day weapons or people that want to easily slam the car imo. Horses for courses really!
depends on what you're doing with the car, if 99% of the time is spent on the street I don't really see the point? some people are tinkerers and want adjustment so fair enough (not me) and others will tell me I 'NEED' coilovers, no I don't, I'll still keep up to them in the hills coilovers or struts. There are some awful coilovers out there the same as there are some pretty poor springs/shocks but I find that the Pro Kit/Koni adjustable setup works great on poor roads where most coilovers would be too hard. 
my 2c :)
 
 

s2d4
What are the reasons behind coilovers having a harsh ride?

Spring rates are too high and shocks are generally valved pretty stiff for motorsport.

s2d4

What are the reasons behind coilovers having a harsh ride?

most of the 'off the shelf' being designed for Jap roads, not our patched up roads, and race tracks. :)

s2d4
lol, I'll bite.
In what way are the coilovers setup differently between jap and aus roads?

 
lol you tell me, you ordered custom rates for yours didn't you?
in the same way a setup is different for the street or a track.

So most seem to agree on well matched springs + shocks (Eg Eibach + Koni) have a perfect balance between ride and performance but s2d4 here are defending BCs so much I'm quite eager to try them out one day on someone's car with a good "street" setting. And s2d4 is not the first person to advocate BC's for street either. Difficult to assess as someone mentioned earlier I think that comfort is subjective :/