Not too sure if it has changed much for cyclists since there are a lot more cyclists now then back then, ie before the congestion charge was introduced. In fact, the boss of Addison Lee thinks cyclists should pay the charge too!! Traffic in London was and is dangerous. The roads are narrow and most drivers are aggressive.
The congestion charge might cause some drivers to drive into central London only after 6.30pm or on weekends, but it's not making that much a difference, if you ask me.
Even today, on a Sunday, there was obviously a lot less traffic when I cycled into central London, but that didn't stop most cars to speed up irresponsibly and putting me in danger. I could count three occasions when I was put seriously in danger by 1) an Addison Lee car overtaking me dangerously and almost hitting my bicycle despite oncoming traffic on the other lane, 2) a private car not stopping and ignoring a STOP sign at a roundabout (while I was in roundabout) and 3) a bus coming out of the bus spot without using the indicator in advance while a car was overtaking me from the other side. And that was all in just one day.
So, it is more a matter of having aggressive and reckless drivers on the road then having a few less in traffic. Sadly, this will not change unless something drastic happens like banning traffic from certain parts of central London, providing better alternative travel options and police introducing more checks and fines.