We have Amex Gold (now upgrading to Platinum), & Qantas Visa but have not been able to use for any of our LONG trips. 5 months in 2008 and 5 months in 2010. From what we were told, they only cover you for (from memory) up to 2-3 months AND each company has different rules as to how much you need to spend on the card to be able to claim. Usually about $250 prior to dpearture. Others you need to pay the whole fare on the card. Call the company for info over the phone and ask them to send the information in writing! Also look out for age restrictions, pre existing conditions etc. Non payment if you leave things in public transport etc.
Another way to go is - talk to your normal health insurer, get a quote from them (they are usually very favourable) and consider puting a halt on your health insurance for the time you are away and using that money to pay for your travel insurance. Health insurers will allow you to do this. You need to do it before you leave and let them know the minute you arrive back in the country to restart your normal health insurance. From memory, you can do this for up to 6 months. Howevere you do not get membership credits for the time the policy in is in abeyance. When you put a temporary stop on your health ins. you don't necessarily have to use that company for tvl insurance. You are at liberty to insure with whoever you choose.