I've been using it for five days now, and it's fantastic. It replaced my 3rd gen iPod touch (non-retina), so I made a big jump. My eyesight is good so I can see pixels on the display, but that's if I make an effort to notice them. For the first time in my life I can view a screen without seeing a jumble of glowing squares. Add to this the huge improvement in contrast and saturation.
Of course, it's fast (not as fast as the iPhone 5 I played with at the Apple Store), has a decent camera, Internet seems faster or more responsive. Mostly, I use it as a PDA heavily using Reminders and Calendar, but also for Music and web browsing. In almost way that I use it it's improved. Siri is a nice addition since it's pretty accurate with my voice at least, and it's faster to use to create new Reminders and Calendar entries than typing them manually.
The only downside is the lack of ambient light sensor; it's annoying but I can live.