It's worth looking at the people who attend ISO working group meetings and examining the companies that they are from. Presumably, if their company sends them along to the meeting, then their company must be getting some benefit from it. At the ISO meetings I've attended, I've met people from IBM, HP, Philips, BT, NEC, TÜV, Fujitsu, OKI, as well as various universities and consultancies.
You also have to look at the sheer scope of ISO 9241. As you’ll know from the Bluffers’ Guide, it covers everything from display screens to form design. The hardware parts of the standards - the parts that focus on keyboards and screens -- are used and followed by all the main computer manufacturers. The software parts are excellent sources of best practice in the field and are enshrined in health and safety legislation in many European countries. ISO 13407 – soon to be renumbered and brought within the ISO 9241 fold -- describes a full user centred design process and is part of many companies’ ways of working, especially in Europe.
You also ask the question, “Does it help?” In user interface design, there are many conflicting viewpoints about good practice. Standards, especially International Standards, provide independent and authoritative guidance. I’ve noticed that some developers are more likely to listen to my recommendations when I tell them that they are based on international standards than when they are based on the opinions of a single "guru" in the field. In larger companies, I’ve noticed that publicly available standards are an enormous help in persuading managers to shift their focus from technology to user experience. Standards can be used to encourage consistency, promote good practice, act as a basis for common understanding, ensure an appropriate prioritisation of user interface issues and support the fulfilment of legal requirements.