I strongly prefer the invisible scroll bars in about 95% of cases.
I'm either using a trackpad or a scroll wheel on my trackball. In the former case, it's trivially easy to jog the screen up and down or sideways, to see if there's missing content. In the latter case, yeah, horizontal scrolling is challenging, but I'm going to be on a widescreen monitor, so my response is to fill more of the window with the content (quick shout-out to Moom, which makes this easy).
It very occasionally happens that I won't notice that there's more content available, and scroll bars would help with that. But I'd rather optimize for the usual case, and get the cleaner lines and extra real estate which go with it.