Had the fluids changed for my ride recently at EZ-Lube.
I asked them what coolant they use. The guys showed me the coolant pre-mixed and stored in a carboy and told me that that was the cooland they use for European cars; it was yellowish in color. Though I was a bit skeptical, I had no choice.
That was about 10 days ago.
Haven't had any problem till now. The temperature stays close to 90 degress C even for highway drives.
The first two offer free shipping once you spend $50, which is pretty easy.  The first and last are sponsors and advertise here, too.
They all have the genuine thing, which is all I would put in my newer Benz. As the other post mentions, it's pricey, but hey, the car you're pouring it in isn't exactly a Ford Focus, after all.
And re: the other/other post, in all honesty I'd be draining and flushing mine first opportunity. MBZ coolant is purportedly blended not only to extend temperature ranges, but also to lubricate the water pump and prevent corrosion in the newer (post '89? I can't recall right now) vehicles. Since they engineered, designed and built the engine and took the time to specify a proper coolant for it, I know I wouldn't trust the yahoos at Jiffy Lube to know better.
Do a search on Zerex G-05 at mercedesshop.com Shop Forum. Lots of discussion about it there. Some people have done quite a bit of reasearch on it (ingredients, talk to mechanics) and consensus is that it's the same as MB OEM coolant. I buy mine for $10 a gallon at local auto chain.
BenzWorld.org forum is one of the largest Mercedes-Benz owner websites offering the most comprehensive collection of Mercedes-Benz information anywhere in the world. The site includes MB Forums, News, Galleries, Publications, Classifieds, Events and much more!