Michael Carter
Veteran

Just got one of these... $140 at Amazon.
I've got a multi-cam "charity" shoot coming up and decided to add my D7000 to the camera package. Needed a loupe, gave this a try.
I gotta say - for the money, I'm way impressed. It's big, it doesn't use BS sticky-magnet-craprings, it fits my screen with or without the battery grip. It's got an adjustable diopter... and, get this - a shutter for the eye piece. The magnifier flips up if you want to see the screen usual size. Big honking huge rubber eyepiece. And it looks pretty dang cool too. Whole thing pops off with one set screw. Adjusts without tools. Image is nice, way less distorted than my Cavision.
Only issues - it's a little big, you need to adjust it to at least hit the sides of camera buttons. And though it has a rubber frame where it meets the camera, there's a 32nd or so of air - which sucks in harsh sun. (My current viewfinder has a similar gap, so I bring a dark cloth from my view camera days). But for this price, I'll glue some black foam or something to it.
For DSLRs, I'm not a monitor guy, unless there's a crane involved. Hate the delay. I can focus like a champ with a loupe though. I won't shoot a DSLR for video without one. I'm getting another of these.
Seriously, if you need a loupe that will fit all of your cameras, it's simply an insane, insane bargain. No sticky frame things that melt off on a hot day. Did I say "adjustable diopter"?
I always love a cheap date. Take a look at Amazon. They're all over ebay as well.