I've the Ldeica 1.4x and the third party 1.25x. The third party 1,25x works pretty good, but it does not come with the nice string. With this tiny stuff, Leica's string help a lot for taking it on and off.
I piad about half of the price of the Leica version. But I'd not recommend it. Not something really significant, but purely from the stand point of supporting Leica. This could be totally irrational. I'm glad that Leica managed to survive. It's sad when thinking how many high quality, legendary camera/lens companies diminished or is diminishing due to the lack of profit.
Back to the usefulness of the magnifier. Yes it's very useful, but you can only use it for 50mm or longer. 35mm frame is visible, but really uncomfortable. You need to get your eye ball into the VF to examine the 35mm frame line. It's very very useful for 90mm and 135mm.
The 1.4x is especially useful since you can then get the 1:1 with both of your eyes. It's expensive, and it really hurts, but I'm glad I have it. what's the word for that? byte the bullet, yes.