My two cents... I have a house full of Aqualung and Apeks, with only a few ScubaPro regulators. So my knowledge and examples use Aqualung simply because I am more familiar with specifics.
In the US (other regions have different policies) the Aqualung warranty and Free Parts for Life program look to have identical annual service requirements. There is an annual service requirement for either an inspection (Year 1, 3, 5, 7, etc) or an overhaul (Year 2, 4, 6, etc...or as needed based on the inspection). Both also both apply only to the original purchaser. So if you maintain the warranty requirements, you will keep the Parts for Life. Failure to meet the annual service requirements for the Parts for Life and it looks like you are no longer covered under your warranty (there are some legal exclusions/conditions for the warranty...not sure what states those may apply in).
Wether a dive shop charges the same amount for an inspection versus an overhaul is another issue entirely. I have seen some charge the same price for an "Annual Service", others have a different cost structure for an "overhaul" versus an "inspection", and I have seen one that performed the annual inspection at no charge. What you pay definitely depends on the store you go to or frequent.
If you are not the original owner, or you have skipped an inspection, by the rules you are now out of the Free Parts for Life programs (and also out of warranty if I read the user guide correctly). Do you even need to perform an annual inspection or overhaul service? There is the "If it isn't broke don't fix it" group who feel that as long as the IP is constant and everything is working, why mess with it. There are also those that want the equipment serviced every year.
If you do get the equipment serviced every year...is the equipment actually being properly serviced per the manufacturer service specifications...even by factory trained technicians?
One shop told a customer that the factory IP (or MP in AL speak) was low on their new Legend regulator. They stated it came from the factory too low and needed to be set to 145 psi so they adjusted it. Problem is, for an AL Legend an MP of 145 psi does not meet the factory specs. The factory specs call for an MP between 116 and 130 psi and the new regulator likely met those specs prior to the technician "fixing" the regulator.
Lets take setting the 2nd Stage opening effort... Some technicians adjust this using an inline tool. Per manufacturer service specifications, this would be an incorrect procedure. On the XTX and Legend (and other models) the inline tool is used only to set the lever height while using a special lever height tool and this lever height tool is unique to each second stage model. Changing opening effort is made by making adjustments to an adjustment screw that fits within the cam.
So if you do pay for your annual service, are you actually getting the service and adjustments the manufacturer has specified for your regulator? I once watched a "factory" authorized tech overhaul some regulators. At no time during the reassembly was a torque wrench used...I am not sure if a torque wrench is even owned. How important is that? Well, some say they can get the correct torque "by feel", other say the exact torque is not that important, and others use a torque wrench because 27 or 40 inch pounds is a lot less than most people think.