There are other websites as well, that supply OEM/OES parts that are great for the DIY owners. They also carry aftermarket stuff as well, for those who want aftermarket, so be sure you know what you're looking for. Maybe there should be a stickied thread on reliable suppliers.
I agree with Nhz, the case for window regulators I am in the same boat, also my aftermarket blower regulator is 2 years strong now with lifetime warranty too. For me, some parts are okay as aftermarket that is not vital to safety, performance. If it works, then it works. The plastic oil tubes, totally true. Older W140s had metal tubes too, it just shows you that they cheapened some stuff to save $ perhaps. They're ~$24 a tube for the metal ones, you do the math, 24x16 tubes... very pricy for some oil tubes. (MB tubes ~$15 per tube)
But as Stryker said, some parts you should not go aftermarket if you want an S Class performance/safety. Brakes, suspensions components, vital engine parts, windows, and maintenance items I consider OEM/OES quality very important. Do not necessarily have to get a MB branded one, but find out the MB original manufacturer and buy the same part to save money. For example, I just replaced my OEM wiper blades this weekend, they had a MB logo on the plastic tab where the blade mounts onto the wiper arm. The same Bosch replacements were cheaper, exact same part aside from the plastic part containing the MB logo.
As for Rock Auto, I used them once but had to cancel my order due to their pricing. I needed a rear wheel speed sensor, it was about 240$ from Rock Auto if I remembered correctly. I couldn't find any other supplier of the same part so I ordered. I got curious as to what the OEM price was and checked the part numbers on genuinemercedesparts.com it turns out the MB part was cheaper! It was like $170 or something and so I ordered that.
Also, just to comment on OEM windshields vs aftermarket. I had replaced my mom's windshield (honda accord) last month with an aftermarket glass from a local glass shop. The difference is not noticeable from outside, but when you look from inside, the edge of the glass distorts the image, if you know what I mean. You get this sort of fish eye distortion around the outer edge of the glass. The original didn't have this affect. The rest of the glass is still fine, but things like this bother me and in cases like this, I would go OEM. I'm due for new windshield on my w140 too, too many tiny rock chips that the glass looks dirty.