Struts are shocks and more. In the front, it's design is such that it becomes a more integral part of the suspension, performing a few functions that other suspensions use separate components for, such as a spring support and suspension axis, is the main steering axis, and also incorporates the shock absorber.

As others have indicated, this can be a relatively easy DIY job if you have moderate mechanical ability and some "auto common sense". I changed out struts/shocks on my 1990 300E last year. In his "Mercedes-Benz E-Class Owner's Bible", I believe Stu Ritter indicates useful life as something around 90-100K for the rears and up to 125K for fronts - perhaps longer than the average american equivalent. As Tom indicates, they will wear slowly and loose dampening effect. In the front, worn struts also affect steering angles. If you find oil leaking from one, that is a clear sign of failure. While I believe the German philosophy is to replace no part before it's time, I prefer to replace shocks/struts as pairs to maintain balance in the suspension, and once I get into a particular type of job, I don't want to come back 6 months later to "do the other one!

My 1990 has been mostly a "Garage Queen" (never out in snow & salt and not a daily driver) and now has 98K on it. Last year I noticed the rear felt more soft and bouncy than I thought it should when I had passengers in the rear seat. One rear shock did show a very slight trace of oil where the piston protruded from the shock body. When I replaced the rears, I did the subjective "comparison push test" between the old and new. Both original shocks were slightly easier to compress than the new ones (DUH!) and one of the originals was softer than the other. Result - improved ride/response at the rear.

While the front struts may have theoretically had a few more miles left on them, I had made a couple other minor suspension upgrades, such as "Sport" sway bars and bushings and new 8-hole rims with tires that matched the O.E.M. issue in construction/handling, and wanted to get as close as possible to that "new feel" in the suspension. No signs of leakage on the front struts but they also did feel slightly softer in the compression test. I used the "Comfort" version and have seen it stated that the Bilstein replacements are slightly more firm than the O.E.M.s. Don't know if that is true but now the ride is more firm, balanced, cornering is flatter, and steering response is improved.
OliverB