I used to inject the patches. I thought it was pretty decent, or at least I kept buying them. Its a cold high given its massive selectivity for the MOR (kappa and delta agonism contribute to warmth and mood lift), but I certainly wouldn't describe it as unpleasurable. There's a dreaminess to it, and perhaps a cool neon artificial warmth reminscent of a fluorescent light bulb. Pretty much no mood lift, but at the same time, no irritability like with oxycodone (which is mediated by the kappa opioid receptor). I would say fentanyl gets a solid ok. Perhaps neon would be the way to describe the high...its like a blue/green neon light -- the neon tubes generate light and little heat, but the overall feeling is cool, electric and artificial feeling. I remember the first time I tried a fentanyl patch (75ug/hr) with no tolerance. It was too strong and I was fucked up for days, vomiting, trying to stay as still as possible, though I remember smiling at my pillow at one point.
Part of it (years later, when I was injecting the patches) was because I was on huge doses of methadone at the time (380mg) so I required very strong narcotics to break through. I would inject entire 100ug/hr patches, which contain around 17mg of fentanyl (which is about 1700mg of morphine).
Its crazy that I am still alive, and even crazier that i'm, generally speaking, not an active opioid addict any longer (I dabble sometimes but nothing serious, nothing intravenous, and when I do get caught up it never exceeds a few weeks and is of low severity).