I did upgrade one iPad mini 3 without problem. This upgrade was GOOD.
However, another iPad mini 3 was a NIGHTMARE. This upgrade was UGLY.
Everything started when I could not get the upgrade to work, so I contacted Apple support to assist me. Two hours later, after resetting everything (from Wi-Fi to All Settings), trying to restore the system (3 times as if this were a lucky number), and changing cables for there was nothing else to blame (and just in case, and because all this was happening while connected to my iMac for upgrading thur Wi-Fi was working), I was directed to go to the AT&T store to ask for a replacement, if possible. At this point the iPad was totally block or in the middle of the upgrade.
Here comes another two hours of figerpointing between AT&T and Apple, and myself trying to understand how did I get into this situation. Anyway, I went to the AT&T store to return the iPad - because it was purchased a week ago. The AT&T rep indicated that I would be charged a restocking fee of 10% the value of the iPad. I said go ahead, at this time just wanting to get all this behind me, but at this same time he turns on the iPad and noticed that this was not working; that this was blocked still trying to upgrade to 8.4. He then told me that "this is a known issue" after the release of this upgrade and that I needed to go directly to the Apple store to ask there to "unbreak" the 8.4 upgrade before he could even accept the equipment. He told me that this is what Apple requires AT&T to do in these situations. Well, in the Apple store, after 20 min waiting, and two reps been very courteous and (apparently) understanding, I find myself going thru the explanation and the three times trying to restore the system (once again), as if there was no record of my previous two hours with Apple technical support. At the end, the technician denied any known issue that this upgrade may be causing with AT&T devices, and gave me the option for a replacement. Of course, the replacement is not a brand new equipment but a "refurbished" or "reconditioned" one (whichever term creates less liability for the replacement is not new).
At the time of this note, I am awaiting the "reconditioned" equipment, which may take "a couple" of days. So, if you ask me, I would tell you to be very cautious with this upgrade, and do all the backups needed in this cases. Just in case this upgrade turns ugly.