My 2 cents worth of quick impressions:
Weight: It is just as light and comfortable to hold in one hand. Couldn't really tell the difference holding it and the original Mini in both hands
Display: A bad piece with uneven color tones notwithstanding, the retina display is so easy on the eyes and really sharp like the iPhone. The difference between the original Mini and rMini is very noticeable when you zoom in to read. I couldn't read for very long without my eyes feeling fatigued on the original, but it has not been a problem on the rMini so far
Speed: Haven't run any processor intensive tests yet but it has been very responsive so far, perhaps a tad more than the Air. I couldn't detect any stuttering in my use and it hasn't crashed on me yet with 5-6 tabs open in Safari.
Speakers: Sounds just like the original Mini, and a little inferior compared to the Air's speakers. The difference is somewhat akin to that of the 13" and 15" rMacbook Pro speakers, in that the rMini's speakers sound a little tinny compared to those of the Air
Obviously it's only been a few hours since I've had it so take this with a grain of salt.
vs iPad Air: It is my preliminary opinion that the Air is still the device to get unless you are a) cash-strapped for $100 or b) really need the small size of the mini (for, let's say, a custom car dash install). The reason is simple - you'll be blown away at how little the weight difference is between these two devices. It's like night and day when you look at how different the iPad 4 and original Mini were. The Air truly is an oversized Mini without much of a weight penalty and thus is just as portable in that respect. The larger screen size provides a much better viewing experience, so unless your use case dictates that you absolutely must have a smaller, but not necessarily a much lighter device I would strongly recommend the Air
Hope this helps
