The Nikon D5300 is a swell little camera, but I wouldn't pay $800 for a D5300 when I can get the pretty much identicalMikon D5200 or Nikon D5100 new or refurbished for about half price. I don't see anything significant to make it worthwhile to throw more money at the newer D5300 if you can still get the D5200 or D5100 instead, but if you want the newest, sure, the D5300 is a great camera. For Wi-Fi and GPS< I prefer the iPhone.
Honestly, when a freind asks, a refurbished Nikon D7000 for $649 is a better camera for less money. I'd much rather have all the controls of the D7000 than the fluff of the D5300.
If you're a seasoned photographer who wants more knobs for faster control, go for the far superior D7100. If you don't need the flippy screen, the D3200 and D3100 are essentially the same thing for even less money. All will make the same superb photos if you know what you're doing, and if you're not an expert photographer, no camera is going take any better or worse pictures for you. The finer points of camera performance, like the finer points of a Steinway versus a Bösendorfer piano, are only apparent or significant to virtuosi.
Forget the ancient D90, which is a leftover from the previous decade; the D90 trails these new cameras in every respect, but costs about the same.
Here's another hint: the only real reason to pay three times as much for a Full Frame FX D600 is to get a bigger viewfinder, and to have to carry bigger cameras with much bigger lenses. The pictures are the same!