Getting data seems to be a much bigger problem than the execution path. It seems to be where all the costs and difficulties show up.
FIX data speed is not really a concern unless you need to monitor a lot of instruments. It's just more verbose so there's additional bandwidth and CPU overhead. They have a new compressed format although I don't think any broker offers it yet. Francis
I presume you are talking about FAST? I am not aware of any retail focused brokers that are offering data via FIX at all, compressed or otherwise.
IB has a solution for the traders who want to trade using FIX: trade through the IBgateway. It allows you to submit orders in FIX and retrieve market data through the IB-API (thus hiding the compression/decompression that takes place between the IBgateway and our servers).