It's all BS bottom line, as it's simpler and cheaper to just do an annual refill of freon - cost me $25 to do that, including a bit of a sealant, to isolate a potential leak. This is after being quoted $800 by BMW (service is much more expensive here in Canada), and just over $250 by a shop specializing in BMWs.

Those DIY cans are a good solution. The only thing to watch for, is that you don't run on empty too long, as you can mess up the compressor.