I've received my TerraOnion MegaSD last week and I finally got around giving it a try as I'm recovering for surgery and I'm recovering my fucking senses at last.
It's a big buggy right now, some games like Snatcher have weird pauses where nothing happens for 2 minutes then the game resumes. I have no idea how this was not detected during QA but eeeh.. The team is working hard with numerous firmware updates. The unit itself is well built, I can't say I'm pleased with the fact that you cannot run SegaCD games if you have the 32x, due to various tech limitations from what I have seen on the forums.
I havent run into any issues with Sega Genesis stuff, you have save states, cheat supports, and also support for Virtua Racing Genny AND weird hacks like the super awesome (if you're a fan) of The Ultimate Mortal Kombat Trilogy hack, which is a super hack of UMK3 with the feature of being able to selection game specific version of each character of the MK Franchise, a custom soundtrack, 4 fatalities per character and its not super janky trash at all. It was made with care (tm)
.. The only drawback is that it's for the genesis version, which suffered compared to other ports, but so much work went into this hack. So being able to run this on real hardware is fucking cool.
Loading games is pretty much instant, there's automatic region patching, options for in game reboots and in-game menu options that you can make pop-up holding Up and Start.
I don't know what else to add, the only problem is the price tag but I'd say it's worth it all things considered.
ALSO - damn a lot of segaCD stuff has aged horribly. I think the fact that i'm playing using an OSSC is REALLY making the segacd's fmv limitations glaring.
I also discovered this hilarious fucking bizarre fmv game called WIREHEAD