Paul PC4T asked me for some info about the MFJ-1788x magnetic loop antenna and I have decided to share my opinions so far.
As some of you might know I live in a row house with no possibility for outside antennas. After reading some reviews I started to check prizes from different suppliers and found that I could save some money by importing the antenna directly from MFJ Enterprises in USA.
I do most of my operations away from home but I have had QSOs with MW, I, GM, LY, S5, OH, 9A, HG, LA and EA with the mag-loop and 5w SSB so far. All QSOs were on 20m except the EA station whom I worked on 17m. My main problem is the very high local QRM that I experience at home, most of the time it reaches S7 - S9!
The loop is located 3 - 4m above the ground in a spare upstairs bedroom I use as my shack. The antenna is mounted in a vertical configuration, currently without a rotator. As this is my only HF antenna at home I haven't had any possibilities to compare it to others but the signal reports I receive doesn't seem to bad for a QRP station with an indoor antenna.
Julian G4ILO has written an excellent review of the antenna that you can find here.