Hi All,
Just had this problem of one side taking longer to cool and always warmer than the other side. The L/H side cools, R/H side less so.
Turned the aircon off and cycled L/H side first from coldest to warmest and it responded correctly. Tried the same on the R/H side, this also responded correctly. Decided that, in view of this correct operation, that the hotwater valves were okay.
Then took the car to an aircon shop, and the Tech' confirmed that this problem is normally due to low gas in the aircon system. He charged it with the correct weight of gas, and it is now back to normal.
Some observations that he made in respect to the X5 aircon:

• the amount of gas in the system is not large,
• gas will eventually seep through the hoses given enough time,
• gas will eventually escape from the compressor seals over tme,
• the returning, still cold, refrigerant is used to cool the compressor during the aircon cycle, so the system should really not be run with low gas.

My X5 is 8 years old and I have, to my shame, never had the aircon serviced. So I have had a pretty good run out of it so far.
The tech also put some dye in the system so that if there is a leak, he can track it down.
Cheers All,
Colin.