James,
Myself and a few others in the office use CQG also.
Why don't more people use EBS data? I have no idea. I don't know of any other data providers other than CQG who provide it.
As far as the spot FX market, mister Ed is correct - EBS can't show 'all' volume for a particular pair, as it's only showing liquidity available through its network.
In the spot market, the problem is no one really cares about 'total liquidity', it's 'accessible liquidity' that counts. There is huge numbers thrown around about how much volume is done everyday, but if it's spread across a multitude of networks - OTC, EBS, Reuters D2, etc - it's unlikely anyone is really able to 'access' it for large trades.
EBS are the leading network for FX. Previously Reuters D2 were effectively running a monopoly.
As you'd understand, certain types of analysis run into real logical flaws with FX -
VSA for example.
EBS data alone should be sufficient. Depending on what you trade, you might want to get hold of Reuters D2 also.
This is starting to become outdated, however the link shows which broker is doing majority of the volume for each currency pair:
http://www.londonfx.co.uk/autobrok.html