Just had mine done after being reminded by this thread. Supposed to have something like 750g of refrigerant stuff in the system, but when it was drawn out mine had just over 500 odd g. After 3 years it had gradually lost nearly a 3rd of the fluid. Nothing to do with leaks though as this is just normal over time.
The system was checked for leaks with no fluid in and no leaks were found. The system then gets lubricated, and then the full 750g (or whatever) of refrigerant then gets pumped back in.
On testing it, it blew out freezing almost instantly which was great. It worked ok before getting it done but because the decrease in its performance is gradual, you dont notice the difference until you get it recharged. Apparently the less refrigerant in the system, the harder the car has to work to maintain the low temperature, until it just stops working.
If it hasnt been done in 2 or more years well worth doing in any event i think. Not something you can do any diy checks with beforehand as the machine does everything required.