I got mine last night! I haven't had a ton of time with it, but IMO it absolutely lives up to the hype. The screen is BEAUTIFUL and the different modes really make me happy. I prefer the modes that simulate original handhelds. Playing Game Gear games with GG mode is absolutely what I want those games to look like (although I can understand why other folks might prefer Analogue or GG+ modes). I realized I don't have an extra micro SD card, so I haven't updated the firmware, I'll be doing that soon. I'm generally looking forward to playing more games with it and getting a better feel for it in the coming weeks.
I've encountered two problems though: first the game Chase HQ for Game Gear doesn't display properly. That game normally looks squished on a Game Gear and I *think* it's using the resolution of a Sega Master System. On the Pocket the picture looks zoomed in and crops off important information from the sides of the screen. I've heard of folks having this problem using the Master Gear or Flash cart to load SMS games, AFAIK no reviews have talked about original Game Gear games having that problem but clearly compatibility isn't 100% yet. [EDIT: it looks like the latest firmware does indeed fix this]
Another problem: the design of the cartridge slot on both the Pocket and Game Gear adapters seems like it can easily damage a cart's label. If the label is sticking up just a bit, inserting the cartridge can cause it to sort of peel upwards
as seen in this Tweet. This happened with one of my Game Gear games, though the label was definitely starting to peel, so I'm not inclined to think it's a super-serious flaw.