My 2 cents and you can take it for what it's worth. I sold the Seek handhelds for awhile and I just had to quit. In my opinion, it's a toy at best and we received so many returns on them from unhappy customers that it just wasn't worth it anymore. I've seen the phone attachments and sadly, I don't see a lot of hunting/tracking use for them either. They are for sale all the time in classified ads because they really just aren't very useful.
Don't get me wrong, I want them to work and for something really close, they are ok, but I just don't see enough benefit to justify the cost. I have found downed deer with the Seek handheld but the image quality just isn't good and you've got to be close to the animal. I'd love to be able to sell customers a $200-$400 thermal device that would really work but so far, I just haven't found it.
Here is a video I found of FLIR One with a kid a couple dogs at about 75 yards. https://www.youtube.com/watch?v=1G_S8HhAGSo
If I was going to buy something like that, I'd go with the Leupold LTO Tracker. It still has its limitations and it's never going to replace a $2,000 handheld thermal monocular but it is definitely legit enough to have some serious usages and again, in my opinion, it's a whole other level compared to the units listed above. It does excellent for finding downed animals, especially in the woods or in fields if you have an elevated position. I have customers who have bought them for close range (100-150 yard) hog/coyote spotting and also several guys how use them to see if there are deer at their feeders when coming and going to their stand in the dark. It helps them not bust deer out while climbing in the stand. They are more expensive but I think the expense is worth it for what you get. For the record, I'm not a fan of the new Leupold Quest. It's literally just a repackaged Seek.
Here is my review of the LTO Tracker.