Pros: This affordable, compact adapter lets you play virtually any Genesis/Mega Drive cartridge – from any region – via your trusty old SNES console’s cartridge slot.
Cons: A few games are (tolerably) less loud than when played via an original Sega Genesis console [and – reportedly – a very few titles aren’t compatible at all]. AV signals aren’t conducted via your SNES console’s own cabling but via a separate (included) AV cable that must likewise connect with your TV’s ordinary composite (“RCA”) video and audio input jacks [but RadioShack or other vendors stock affordable AV switchboxes to make dealing with this easy and convenient].
 
Back in the day,  many gamers passionately deemed themselves staunch adherents to either the “Sega” or the “Nintendo” camp. But nowadays retro gamers can easily afford to be more eclectically open-minded.
Prefatory paragraph (feel free to skip ahead):
Some weeks ago I bought Hyperkin’s long-awaited “Retron 5” videogame console; and – initially – I was altogether pleased with that uniquely multifaceted system’s compatibility with Famicom, NES, SNES, Game Boy, Game Boy Color, Game Boy Advance, Genesis, and (provided you optionally connect Sega’s “Power Base Converter”) Master System cartridges. Unfortunately, however, after a couple of weeks my Retron 5 specimen suddenly, repeatedly (even after multiple restarts of the system) failed to function reliably, if at all, with four of its five cartridge slots (the consistently happy exception being the slot for the aforementioned three “Game Boy” formats). Then – equally unexpectedly – my Retron 5 (just one day later) suddenly became largely functional again with all five slots. That said, considering such occasional/potential behavior; and considering the quite visible degree of inauspicious, upward/outward “bending” of several of the metal contacts in, specifically, its excessively tight NES and Famicom slots, I’ve concluded that it’s probably too soon for me to be either endorsing or damning the Retron 5 (for the benefit of prospective consumers). Accordingly, I recently deleted my pertinent product review [something I’d virtually never done before!]; and I’ll wait till much more time has elapsed before maybe trying again… someday. [Even as it presently stands, the Retron 5 does embody various undeniably admirable features; and my hope is that Hyperkin will be continuously striving to improve the presently not-so-good features.]
Meanwhile, the primary subject of this review is a product that appears to be much simpler and fully trustworthy: Retro-Bit’s RetroGEN “Genesis-to-SNES” Cartridge Adapter.
You see, during my Retron 5’s aforementioned “briefly broken” phase, I’d immediately googled alternative means for playing my recently purchased batch of NES cartridges. This quickly led me to Retro-Bit’s RetroPort “NES-to-SNES” Cartridge Adapter [which I expect to receive (and possibly review) soon], which would allow me to play “Nintendo Entertainment System” (NES) cartridges via my Super Nintendo Entertainment System (SNES) console (i.e., my late-1990s “mini” or “junior” version of the original SNES console). And it subsequently caused me to discover the availability of this analogous Retro-Bit adapter for playing Sega Genesis cartridges via my SNES console:

I ordered this product from Amazon.com for $22.52. The thin-cardboard product box contains three pieces: the adapter itself (which, per my hands-on inspection, weighs 2.8 ounces and measures 2 & 7/8 by 5 by 13/16 inches); an approximately 58-inch-long AV cable (to connect the adapter to your TV’s ordinary “RCA” audio and video inputs); and an instruction manual (i.e., a modest, folded sheet of paper with a single page of English text, plus four analogous pages in other languages).
Actually (assuming you’re already aware of the need to keep your SNES console turned OFF whenever inserting/removing a cartridge), the printed instructions are nearly superfluous. For, all you basically need to do is plug the single “minijack” end of the AV cable into the pertinent port on the RetroGEN adapter’s left edge, and then connect the opposite, “three-pronged” end (yellow, red and white “RCA” plugs) to your TV’s corresponding (composite) video and audio jacks. Of course, you also need to plug the RetroGEN adapter’s cartridge-like bottom into the cartridge slot of your SNES console. Finally, you’ll plug an original Sega Genesis (or Sega Mega Drive) cartridge into the top (slot) of the RetroGEN adapter. And then turn on your console.
Thankfully, I found that the fit of this RetroGEN adapter’s “bottom” or “top” isn’t worrisomely “tight.“ In other words, I found it sufficiently easy to plug this adapter into my SNES console’s cartridge slot; and it was likewise easy enough to plug the typical Genesis cartridge into this adapter’s own, upper slot. Similarly [especially in contrast with the aforementioned Retron 5 console’s “viselike” Famicom and NES slots!], it was easy enough to remove this adapter from the SNES console’s slot; and it was likewise reasonably easy to remove a Genesis cartridge from this adapter’s pertinent slot.
This adapter draws its electrical power through your SNES console; moreover, your console’s connected Nintendo hand controller is what you’ll use to play Genesis games. Otherwise, though, you can think of this RetroGEN adapter as being equivalent to a miniaturized Sega Genesis console; accordingly, the audio and video signals are conducted via the included AV cable, not your SNES console’s own AV cabling. Accordingly (in lieu of continually unplugging my PlayStation 2 “Slim” console’s RCA jacks from my present two-way AV switchbox), I’m feeling increasingly compelled to (soon) replace that two-way switchbox with either a three-way or four-way switchbox, such that I can keep this RetroGEN adapter’s own AV cable more conveniently “permanently” connected and selectable via its own designated button on the switchbox.
So, just how nicely does this RetroGEN adapter actually work? Well, it works very nicely indeed – at least with the vast majority of Genesis game titles. [Reportedly, an extremely tiny minority of titles (e.g., Virtua Racing) isn’t compatible.] For example, I encountered no really noteworthy problems with any of the below titles, which I hastily, randomly grabbed from my collection:
Truxton; Viewpoint; Phelios; Thunder Force II; Subterrania; Super Monaco GP; Lightening Force; Pac-Mania.
With each of those games – via my billiard room’s Samsung LCD TV – the video quality was uniformly excellent, and the audio was almost always comparably fine. Oh, I did notice that one or two of those titles weren’t 100% as loud via this adapter (as via an original Sega Genesis console); however, in those instances the discrepancy was so modest or tolerable as to make scant difference to me (and, after all, a TV’s volume control is easy enough for a finicky user to adjust).
I don’t doubt that essentially all of the remaining Genesis carts in my collection will play likewise satisfyingly via this RetroGEN adapter.
The only “control” on this adapter is a tiny black four-way sliding switch (on the right edge). Via that four-position switch, you can play not only North American “Genesis” cartridges but also their “Mega Drive” counterparts from other global regions (including even Japan). From top to bottom, the selectable settings are legibly labeled with white, uppercase letters:
NTSC (North America – the “factory-default” setting);
PE (PAL Europe);
NJ (NTSJ Japan);
PA (PAL Asia)
And since (unlike my North American Genesis console’s cartridge slot) there’s nothing surrounding this RetroGEN adapter’s own upper slot that would hinder inserting a conventionally “Japan-only” Mega Drive cartridge, I look forward to getting at least one such long-coveted, vintage “SHMUPS” game ASAP.
Considering its low cost, compactness, and ease-of-use, this Retro-Bit “RetroGEN” is a product I can pretty confidently recommend to virtually any SNES console owner who’d relish playing – gasp! –“Sega” via “Nintendo.”