I use the Japan Exposures x1.15, which is permanently on my M8 - love it. The rubberised Leica eyepiece surround on the camera grips the magnifier very securely when done up tightly - it's never come loosened.
As mentioned, the Japan Exposures magnifiers have an adjustable dioptre. I wear contact lenses and have excellent vision with them, so wasn't that interested in this feature, but duly adjusted it for sharpness. After, I noticed that the viewfinder was sharper with the magnifier than without - in other words, despite having perfect vision with my contact lenses, the standard Leica viewfinder needs a slight change in dioptre to suit my eyes (those not interested in magnifiers might like to experiment with the accessory Leica dioptre lenses).
I put a drop of weak threadlock on the dioptre thread as it wasn't as tight as I'd like. Since then, it hasn't moved, Just in case, I made a mark on the magnifier so I can reset the dioptre if it moves.
x1.15 doesn't sound like much but it makes all the difference to my M8, at least for me, with the 35mm and smaller framelines filling the viewfinder more and thus being more usable. The 24mm framelines are very tight, but usable if you don't wear glasses (I rarely use that focal length, so not an issue for me).
The 90mm frameline is still a touch small in my opinion, but still better than without a magnifier.
Like all magnifiers, there's a slight loss in brightness of the viewfinder - all magnifiers do this, as you can't break the laws of physics, and more glass further away will dim the view. But I don't find this objectionable, and this has no impact on usability.
I also have a cheap eBay x1.35 magnifier. Compared with the Japan Exposures x1.15 magnifier:
• it has a poorer build quality
• the optics are not as good - more distortion
• it's bulky (more than the Leica or Japan Exposures x1.35)
• the threads are less accurately machined, so it's prone to coming loose
• the 24mm framelines aren't visible, and 28mm are very tight, even wearing contact lenses
• the viewfinder was dimmer - but that's because it has a higher magnification, and I'm all x1.35 magnifiers will have the same effect.
All that said, the eBay magnifier is good value for money, and makes using 75mm and 90mm lenses much easier. I wouldn't want it my camera permanently though because of the distortion (albeit slight) and the dimmer view.
I'm happy using it occasionally when I need more accuracy with 75mm and 90mm lenses.