Hmm, tough one for us in the USA, as we didn't get alot in the way of tuned MB options here.
I will through in my .02 though.
Brabus - Body kits crappy. Engine mods pushed the limits of the motors. By increasing displacement past the safe limits, they compromised longevity. This may not apply to everything they have done, but it does apply to M103 and M104 motors.
Lorinser - Not much experience here, but my CLK had most all the available Lorinser parts for the W208. Body kits, crappy fiberglass. Pedals, over priced for what they are. Exhaust, overpriced for what it is. Wheels, good quality but extremely expensive and they went the cheap route in their design. They don't have specific fitaments, they have a generic fitament and then sell you spacers to adapt the wheel to your car. Some tire centers won't do any work on your car/wheels if you run spacers. I found this out the hard way
Kleemann - My clk had a Kleemann SC on it. It is extremely expensive and the HP numbers Kleemann claim are extremely exagerated. There were also reliability issues, plastic impellers in their external water pump liked to explod causing major motor damage. Believe this issue was fixed on later model designs by going to metal impellers. I won't be buying any Kleemann parts in the near future.
Renntech - Have used a few of their products. No complaints except for maybe the price. Waaaay over priced is more like it.
Have no experience with the other tuners, so can't comment.