w210; Nitrogen sphere replacement; How effective might this be?

[Paul T]

2002 E320S4 (4Matic wagon) with 155k miles: The ride is hard; not bouncy, but harsh, felt more in the after end. Suspension is all original. Shop says small fluid leakage weepage seen at shocks. Some have suggested that the spheres should be replaced, typical at this age. So, I stopped in the the dealer. They drove it, said yes, the ride is hard. But, they just did spheres on a similar car recently, and it only made a minor difference, suggesting that the reward may not be worth the $600 to $700 that the job (spheres alone) would cost.
Any experience in this area?
Thanks for your comments.