Also, for those that don't know, to view these:
Copy the demo file in to \steam\steamapps\<your username>\<Counter Strike Source or Team Fortress 2>\<cstrike or tf>\
Then open CSS/TF2, open the console using ~ (must be enabled in options->keyboard->advanced), then type "playdemo <filename>" - you should get a drop down list of options as you type the demo file name.
Once in demo, type "demoui" or "demoui2" to bring up playback controls. Do note that I've had issues with jumping to certain ticks using the TF2 files.
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