The biggest difference in HD vs non-HD is when you're on winding crappy roads like we have around here. The non-HD's make the car wallow and the lack of firm damping makes the car skittish and have to take the turns at less than speed limit. Almost like worn shocks but not since you bought new. The HD's *DO NOT* make the ride harsh. Anyone who claims they do must have an ass made of glass. I've run HD's on both of my 126's and wouldn't run anything else. They glide over even the crappiest potholed, rutted, washboarded roads and handle so much better on the winding bits. Everyone who rides in my car comments on how smooth and comfortable it is.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

In my 7 years of W123/C123 and S124/A124 ownership I've only installed HD Billys.

Zero regrets. They are fantastic, and my experience has been just as Nate andDiseasel describe.

I've driven W123s with non-HDs. My experience there is also as Diseasel describes.

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1983 300D Midnight Blue
4 speed, W115 intake mani, non-EGR exhaust mani, KKK K26, 3 pc Euro bumpers, Lo-spec Euro headlights, AL129X, hubcaps, custom A/C

1987 300TD Smoke Silver
Euro headlights, thermostat relocation, coolant bypass mod, rebuilt Becker + Jehnert + Helix audio, OEM Oris roof boxes and surfboard racks, Euro towbar

1983 300TD Reed (Moss) Green / Dark Olive MB Tex Euro spec

1983 300TD Silver Blue / Blue cloth Euro spec, OM648 and 722.6, 15" hubcaps, W126 S2 brakes