The OSSC is a godsend for a specific kind of user. Retro enthusiasts who care about image quality and near-zero lag. It'll do component video, RGB video, VGA, handle all kinds of sync types, etc. It's designed mostly for SNES, Genesis / MD, PSX, etc.. but vintage computers work great with it as well, provided your monitor is OK with HDMI carrying PC resolutions and 70Hz. My TV is fine with that, so it's hands-down the best thing out there - very flexible and great image, if you're willing to put in the effort of configuring it properly. It's about 75% plug-and-play, so if that's the priority, it's probably not a great fit.
I've used a few of the generic Composite / S-Video / Component / VGA to HDMI scalers, and most of them suuuuuuuck. I've got a Startech video-to-VGA box that looks soft and often noisy, and a Geffen VGA / RGB / Component to DVI that looks like mush. (IMO, Geffen make complete garbage and don't support it worth a crap either. Lesson learned.)
But, some of the VGA-to-HDMI (or DVI -- same difference) converters work well enough. I have a tiny one I use for guests at work that come in and want to project video from a laptop with only VGA ports. It does the trick, but it's a converter not a scaler (so the monitor still has to scale to the native panel resolution) and I never really scrutinized the video quality. It technically worked, and I was happy with that.
So, for the Aten... only way to know is to try and see. Or I guess there are reviews on some of those fancy websites now. So two ways.