I bought the manual from ToadHall book a few months ago, and I find it somewhat cryptic.
As per the other replies, it attempts to condense the many variations of engine displacements and fuel combustion types that encompasses the W124 model. This is what makes it so cryptic. If the pictures that accompany the step-by-step procedure depict an engine different from your own, you have to "guess" the general area where the part may be found, or extrapolate what it may look like. From the pictures I've studied, they favor the early 6-cyl versions. But at least they have separate sections for servicing diesel and gas engines.
As a DIYer, I have gone ahead and performed repairs on the car WITHOUT the manual, and then referenced it later. In a lot of cases, my procedures were better than what was recommended in the book. On the other hand, one time I did way too much disassembly when replacing brake pads, to the point of having to go to the dealer to get them to fix my mess. When I later received the manual, found that the task was far easier to do.
The manual does attempt to rate the skill level required to perform each service on a scale of 1 to 5 using a representation of 5 spanner wrenches. The difficulty increases as each wrench is highlighted. As far as servicing my W124, I consider myself a very novice individual, but based on my experience with this manual, I would be inclined to put the skill level of the first two higlighted wrenches in the category of "knows what a wrench IS".
It does have some good points though. At least now I know what this black doohickey over here is called, and what that silver round thing actually does.
Since the interior and exterior components of the various W124 models are more similar than the engines, the manual guidelines for servicing in those areas has been useful.
I think the manual serves as a basic guideline for someone who has already has some skill with servicing a W124, and has a pretty good idea what they are getting into.
As far as doing your first timing chain replacement ever using the manual...well, you'd better get a forum member to help.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car