I've heard mixed opinions on them - some people say they work like a charm, others say they're useless or even dangerous. Are there any good free ones? Would any of them improve speed or boot time on an older PC any more than disabling some of the junk that gets loaded at startup and uninstalling some older, unused applications, like I've been doing today?

I use the registry cleaner that comes with CCleaner.

But yeah, registry cleaners are very dangerous. I pick and choose which keys for it to delete, one by one.

Okay I'm lying, I just let it delete anything it wants. But you're SUPPOSED to pick and choose them one by one.