Hi,not rock hard but annoyingly bouncy and on hard bumps you feel it. What is happening is the tires are absorbing most of the bumps.
If the boots have gone,the shocks won't last much longer.
A quick test is to lift the car up and onto stands, let the wheels hang down.
Undo the flex line into a sphere. it will spray oil out .As the oil lands on the ground (or piece of old cardboard) look at it carefully. if is flat and is still ,the spheres are OK. if it is bubbly like oily coke, the spheres are shot. it only takes a second to do this. The bubbles are nitrogen from the blown spheres.
It takes about 30 minutes to swap the spheres. You need a 13mm socket to undo the mounts, a 17 mm open ender to undo the flex line and a 11mm flare nut spanner to undo the pressure line.
Let the pressure go on the valve by undoing the bleed nipple. I stick a meter long piece of plastic line on the bleed nipple and aim it into an old bottle to dispose of it later.
Once the flex line is off ,I poke a piece of welding wire into the sphere (depends if it has a right angle fitting or banjo bolt) . If the wire goes easily into the full depth of the sphere,it has definitely failed as the diaphragm has blown and the nitrogen has escaped into the oil circuit. No nitrogen,no spring. on a good sphere the wire only goes in a short distance. Poke it in carefully! you don't want a sphere blowing oil at you.
The blown spheres also have lots of oil in them ,good ones don't.
New ones are $300 each and the oz wreckers http://www.uniqueautoparts.com.au/ sell used ones for $100 .
Once you have fitted the new ones and made sure all the lines are secure,open the bleed nipple fully.
Change the filter on the reservoir,and if the oil is really dirty ,remove the reservoir and wash it out . Check the soft suction line and if it's a bit dodgy looking,replace it.
You need at least 3 bottles of the oil, all merc dealers have it ( don't pay any more than $15 per bottle, try http://www.imparts.com.au/ if you your local dealers want more ) .Put two bottles into the reservoir and start the engine,let it idle for a minute.
This will blow all the old crap out of the pressure line.
Stop the engine.
Place stands under the axles so the cars weight is on the springs, Old oil should be squirting from the bleed nipple.
To make the next step easier, I put 4 25 liter buckets in the back of the car. I fill these with water. This pushes the springs right down so the shocks are bottomed out.
Put the last bottle of fluid into the reservoir and start the engine.
Watch the hose on the bleed nipple and as soon as the oil runs without bubbles shut the nipple off. The car should now rise to ride height.
Check the spheres for leaks.
I usually degrease the cars underneath and sit them on a piece of paper .if the shocks are leaking ,this will now become apparent as the oil leaves marks on the paper.
Empty the buckets..
If the shock boots are missing ,the dirt from the road wrecks the lower seal and this kills the shocks ($700 each in Oz )
The oil leaking down also destroys the bottom ball joint as it drags dirts into them. You will then a banging noise all the time .
I don't think mercedes still sells the boots but even a bike tube zip tied into place is better than nothing.

If the height valve leaks the car will settle everytime you stop and pump up when the engine is started . Valves last a fair while but if the oil is allowed to remain dirty they wear out fast.