I've always preferred modding a system over converters. It just adds another step you have to take when you want to play a game. Here are some alternatives to adapters:
Turbografx-16: I added a switch, allowing me to switch between PC Engine and Turbografx-16 modes. It seems to work perfectly, but installing it required a lot of wiring. I hear the import adapters for the TG-16 can be a little twitchy, whereas this works all the time.
Genesis 3: The Genesis 3 already has a slot large enough to take Japanese Megadrive games, so no modding is needed there. To get past the software lockout, I sort of had to use trial and error since my circuit board was laid out totally differently than the examples on GameSX. I rigged it up so that all you have to do is hold down the reset button while powering it up to make it enter Megadrive mode. Seems to work on all games with no hassle. It was worth the effort, since the mod is just one wire.
Sega CD: I've used SCD_Conv and wasn't happy with the results. Some of the games I wanted to play wouldn't work, and I didn't want to have to burn copies of all Mega CD games. I also didn't want to pay out the nose for one of those adapter cartridges. I ended up adding a multibios instead. With the flip of a switch, I can change it from Sega CD to MegaCD modes, and it works with every game I've tried with it. I also tried making a single universal BIOS by altering the boot code, but I discovered the code I'd need to change is compressed and I don't know the compression algorithm.