Before I ever read the research I just decided to give it a try because my IBD makes absorption of this mineral more difficult.
I made my own magnesium oil, using MgCl2. At first I sprayed too much because within a day I was having diarrhea, which is a side effect of having too much magnesium at a time. It relaxes the bowels and makes transit time faster. Tell me, if it doesn't absorb, then why did I have bowel symptoms? And don't try telling me it's just in my head, or I ate something bad. I know my own body.
I've made some of this cheap magnesium source for friends and the same thing happens to them if they spray on too much, or too often. If you look at any website which supports transdermal magnesium, they will give you the same caution (which I wish I knew about before I went ahead and did it). There are competing industries online and yes the research industry is mostly privately funded these days (unlike pre-1985), mostly by pharmaceutical companies, so you won't always get honest answers. On the flipside, you have the natural health industry calling it a miracle and selling 250mL for $20 USD. If you can buy MgCl2 in solid form, you can easily make it yourself for $5/bottle.
The stuff works. You just have to rub it in the places where skin is thin, like under the forearms, your abdomen/lower back, behind the knees, and the armpits. I spray it on first thing in the morning, wait about 20-30 minutes, and then get a shower. I'm sensitive to chloride/chlorine so I can't keep it on my skin all day or it'll make me itchy.