I flew in 744 Y+ (real thing, not a J seat) before on JNB-SYD. My ticket was actually a Y seat sold in a 2-class config, but the plane operating our flight turned out to be a 4-class 744. This means that the J ticketed pax got to sit in both F and J seats, whilst the Y ticketed pax got to sit in both Y+ and Y seats, with the better status Y pax getting the Y+ seats (but of course only Y service).
There is a few more inches of recline in a Y+ seat, plus a little bit more width. It's certainly not a J seat, not even close to a Millennium J seat (although to much chagrin quite similar to the new J being rolled out on Jetconnect 738s and newly fitted 332s on trans-continentals), but it is more comfy than a Y seat, although certainly a few people are in disagreement here.
Food is better in Y+. It's not quite J standard, but it does step up from Y. I think they also do sandwiches made to order in Y+ that you can have between meals. The bar selection (if you drink) is the same as J, including real Champagne (non-vintage, of course) instead of simply sparkling white (of which the standard drop in Y is Jacob's Creek).
Not sure what amenity kits they give you in Y+ (because I think Y just give you the plastic pencil case now rather than the older passport pouches).
Should also be quieter than the Y cabin given that there are only 32 Y+ seats. The bathrooms, however, are located behind the Y/Y+ curtain.
If you're rather tall, I think I'd recommend selecting a seat other than in the front row. The solid bulkhead in front of Y+ means slightly less leg room compared to the other seats (i.e. where you can slip your feet under the seat in front of you. When everyone's reclined, unless you are very tall it probably makes little difference (although when you are facing a seat which is fully reclined down, getting out becomes very awkward).
Of course, you could very well luck out a 2-class 744 and get a J seat for your Y+ ticket!
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