Yep, spent a lot of time last night trawling the web trying to understand this.
Seems that the Fiscon unit is really good, but two things putting me off are the price and the fact that it doesn't use a hidden mic like the OEM setup. I even had a look at the Dension, but that does not seem as well integrated using the CD port.
With the OEM setup with A2DP seems like, as you say, they are not very good. I am also led to believe that the RNS 510 can store music on the HDD making A2DP virtually obsolete anyways.
I have been reading about issues with the Iphone 4 and the 729B, have you heard anything about this?
It's crazy that you can buy the newer kits for under £200 and the older kit costs £400.
If I had to use the Fiscon, do you think there is a way I can mod the mic to use the OEM unit? And secondly, where is the best place to get them? Cheapest price I have found is £330.