I had a W140 S500 and man, that was SOLID. My LS400 feels a bit flimsy in comparison. I think the W220 S-Class is not in the same league though.
I've never been in a 430 but the 400 is nice. Reasonably comfy (but the W140 was more so) Quick enough without being acary, good ecconomy too. All in all a good all round car.
Could do with a tad more space, a bit more power and a more "special" feeling. Then it would be perfect!