Finally got a chance to play with the 3-methyl-butyr-fentanyl. Product seems to be on point; it comes as a shiny white powder (similar to the powder butyr-fentanyl HCl) and is of high purity.
As for the chemical itself, it's almost exactly like butyr-fentanyl, but more potent. According to Wikipedia, adding a 3-methyl group to regular fentanyl makes it 10-15x stronger per dose, and I think a similar comparison can be made with its butyramide analogue. I would definitely say that 3-methyl-butyr-fentanyl is about 10x more potent, possibly a bit more than that, but from what I've gathered it is closer to 10x rather than 15x the potency. Obviously, one should start at a lower dose to play it safe. The subjective effects are pretty much indistinguishable from butyr-fentanyl, so it's still super euphoric, and so far I haven't noticed a difference in duration.
Just a note to anyone considering buying this: please be extremely cautious. Normal butyr-fentanyl is already quite potent and has killed plenty of people, so I can only imagine what will happen if this chemical gets into the hands of your average, idiotic teenager. On top of that, I think the increased binding affinity means it possibly is even more addictive, raises tolerance even more, or even is harder to reverse in case of overdose. Take it with a grain of salt, but I've read before that (non-butyramide) 3-methylfentanyl cannot be reversed by naloxone, and I'm pretty sure I've heard someone say the same thing about butyr-fentanyl too.