Several of them (especially the later ones) came up with an error message along the lines of "This cartridge is for use on NTSC systems only". Some didn't change at all, specifically the third party games. However, some of the very first Genesis releases showed their true colors - they reverted to the Mega Drive games that they were originally.
Streets Of Rage came up as Bare Knuckle. Truxton came up as Tatsujin. Ghouls 'N Ghosts came up as Daimakaimura. The Revenge Of Shinobi came up as The Super Shinobi. Mercs came up as Commando II. Thunder Force II came up as Thunder Force II MD. ESWAT: City Under Siege came up as ESWAT: Enhanced Special Weapons And Tactics. The only third party game that changed was Wolf Team's Final Zone - it changed back to Final-Zone Senki Axis (aka Axis FZ). Several of the games didn't have altered title screens, but the English text was changed to Japanese - Forgotten Worlds, Flicky (besides text, the character names were also changed back to the originals) and Midnight Resistance in particular.
The one game that was changed the most, though, was Mystic Defender. It reverted to its Mega Drive incarnation, Kujaku Ou 2 (based on the anime of the same name, otherwise known as Peacock King). Numerous things were altered - the color of some of the characters, the text was changed to Japanese, pictures showed up in the intro (which was shortened considerably), the Thunder Dragon that appears when using Thunder Dragon magic was replaced by some kind of six-armed warrior, the original Japanese names of the characters were shown in the ending, and Joe himself becomes Kujaku - all pictures of him in the opening, between level screens and the ending, as well as his in-game sprite, were changed to show him wearing robes - which Kujaku, as an apprentice exorcist, wears normally. The original Japanese title screen appeared, and there was even a stage clear theme that played when a level was cleared.
Amazing. I've had Kujaku Ou 2 all these years, and I didn't even know about it until I tried this out... and I found this out just after I bought a copy of the actual Mega Drive cart. Oh well. It doesn't matter, as I'm happy to have both of them (and for scans of the Kujaku Ou 2 box and instructions, as well as a side-by-side comparison of Kujaku Ou 2 and Mystic Defender, check out the article The Twisted Saga of Kujaku Ou).
It's quite the thing, I'll say that much. If you have a converter and any of these games, but haven't tried this yet, give it a shot. You might be surprised at the results. For that matter, try some other ones that you may have- if they work, let me know.
For reference, here's a list of games that didn't work or showed no changes, out of the ones I tried:
Shadow Dancer - no changes
Shinobi III - doesn't work
Strider - no changes
Altered Beast - no changes
Last Battle - no changes
Alex Kidd In The Enchanted Castle - no changes
Golden Axe - no changes
Alien Storm - no changes
Ristar - no changes
Ranger X - doesn't work
Phantasy Star II - no changes
Phantasy Star III - no changes
Castle Of Illusion - no changes
Target Earth - no changes
MUSHA - no changes
Valis III - no changes
Thunder Force III - no changes
Lightening Force - doesn't work
Bio-Hazard Battle - doesn't work
Flashback - doesn't work
Splatterhouse 2 - no changes (damn!)
Splatterhouse 3 - no changes
Each version's title screens
Exploding bodies vs. flying ones... I'll take the exploding ones.
Normal guy vs. mutant... which one would you pick?
The title screens
The aforementioned weight
Alex and the Queen - NAKED!!!
System Shock Issue #4 has a really good article on both of these... why not stop by and check it out?