I've tried the "Kung Fu Flash"- it's a software defined cartridge that is cheap- just a single STM32 and can do pretty much everything. I bought this because I'm trying to duplicate the developer experience I see on "8-bit show and tell"- it can emulate the "super snapshot", but not the REU. It's a really nice way to quickly try a lot of C-64 software and games.
https://8bithardware.wixsite.com/website/post/kung-fu-flash
https://github.com/KimJorgensen/KungFuFlash
I also have an SD2IEC: what I've learned is that it would have been useful to get a variant with an extra DIN socket. It's nice but I was never a fan of C-64's DOS and this reinforces it. To mount a D64 disk image you have to: OPEN1,8,15,"CD:MYIMAGE.D64":CLOSE1... yuck..
JiffyDOS (replacement ROM for the C-64) improves this (it's faster and includes a permanent DOS wedge), I bought one- it's on the way. I'm curious to try it with the real 1541 drive.
What got me started on this recently is the "Penultimate +2" cartridge for the VIC-20:
https://www.youtube.com/watch?v=eNGyneXHKJQ
In this case, I basically bought a VIC-20 just to try out the cartridge.