I don't know anything about the M3SE but I can highly recommend the FreHD. I installed one internally in my 4p and it's amazing. I highly recommend this seller:
Find many great new & used options and get the best deals for FreHD Hard Drive SD card emulator for Tandy Radio Shack TRS-80 Model III 4 4P 4D at the best online prices at eBay! Free shipping for many products!
Thanks for the feedback. I've bought a PS for my 4P from Jay and it's a quality product that works perfectly so I definitely trust his work.
The FreHD is $100 cheaper as well, but has fewer features however than the M3SE.
Is there anyone here with hands-on experience with the M3SE?
Yes, they both work well. I use the M3SE mostly, Pete Bartlett is adding WIFI support here soon. One down side it currently only supports LDOS/LS-DOS in either Model III/IV mode. He has mentioned adding support for other DOS variants. Maybe after he releases the WIFI module.