Ok, since there are a lot of people who ask make or watch a demo, I'll explain how to do it once and for all so everyone can see it.
Making a demo:

-First of all, you have to be speccing the person you want to take a demo of.
-Then, when you are speccing them, open your console and type "record demoname" without the "". And place the actual demo name instead of demoname. Make sure that you DO NOT put a space between the demo name and the number if you want to put one. For example. If you want to name a demo: Mcpenco1, don't put a space between Mcpenco and 1 because it will overwrite some demos.
-Then, just watch him. Make sure that if the player you are speccing dies, you target him again.
-Then, when you want to stop your demo, just type "stop" in console without the "".

Uploading a demo:

-First of all, go to http://www.mediafire.com/ and click on upload files to mediafire.
-Then click on I want to upload without an account, or make an account if you want to manage your demos later.
-Then a page will pop out for you to browse your demo. They are saved in your cstrike folder.
-Click on open
-Then, click on start upload.
-Then, click on upload to a new folder (it will start uploading at this point) and leave it until its done.
-Then when it's done, just click on the "copy link" of the first link and you are done!

Watching a demo:

-Click on the link to download the demo you want to watch.
-Save it in your cstrike folder (except if its a folder with mutiple demos, you save it to your desktop and then extract the folder to your cstrike folder).
-Download the demo.
-Open Counter Strike Source without joining any servers.
-Open your console and type "demoui" without the "".
-Click on load and browse your demo.
-While viewing the demo, open your console type sv_cheats 1 and r_drawothermodels 2 so you can see if he is aiming at the enemies through walls.
-When you are done click on stop.

Hope that helps, if you have any questions, just post here and I will reply.