I've fooled around with the Hammer Editor within the SDK for a bit. It's fine, although Valve's decision to have the only closest thing to a user manual is the bare-bones Source Wiki somewhat intimidates average players from mapmaking.
Downloading the SDK is a royal pain in the ass; the download time is horrendous on DSL, and the SDK requires you to run it in Steam, further slowing down your machine.
Overall, if you really want to see if mapmaking is your thing, then go ahead, although expect little in-program help. To be honest, the easiest thing to mod in Source is the config files, as all you really need is WordPad and knowledge of console commands.