well I'm one of the idiots still trading with a MM - CMC for that matter!
I haven't really experienced problems when placing trades on the open market. I've read claims of foul - play on "CFD provider reviews" and such - while it does not appear to be a systematic issue, there are times where I have thought that some amount of stop-hunting has occured. This is probably much more of an issue for illiquid instruments. I agree that it's really too much to have to trade against the market and your broker too. Right now, I'm considering trading ASX listed CFDs.
On the plus side for a MM, they do offer more instruments and leverage compared to say Comsec's listed CFD offering. I probably don't need both, and it certainly introduces an aspect of borrowing to gamble.
Anyways, what really frustrates me is that CMC's client data feed and order processing can have a habit of stopping straight after some important news release / major market spike up/down. eg. The last Fed rate cut a few weeks back led to a data blackout for me of about 1/2 hour. Market trades straight after these events have a higher chance of not getting filled properly / requoted.
For this reason, stop losses are very important or you could end up with a huge loss. Frantically attempting to call up a dealer but receiving the "please wait, we're busy" voice messages does not help whatsoever.
on the whole, I've lost money on CFDs so far. I've probably highlighted their disadvantages a bit more, but overall, I've found their service and platform not that bad to use.
This is not advice. Do your own research.