The MB ones aren't very useful (maybe slightly for certain jobs, but overall the FSM is the way to go, I have it), we got a W123 book in the glove box of one of our cars when we bought it, so technically it was free. 
However, for my uncle's chevy cavalier and our minivan we have both of those Haynes books, and they are extremely helpful. We've done a number of jobs on both cars and saved a TON of money. Brakes...cooling system....pumps.....random little things like that. Way easier with the Haynes book because it outlines the disassembly and reassembly steps. Especially helpful with the minivan since you have to take half of it apart to do any major work. 
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-diesel is not just a fuel, its a way of life- 
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)
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