My point about "the number or the needle position" still stands: a (usually) two digit number is a lot faster to register than text. Your altimeter says "5.8", not "five point eight"
Usually, perhaps. Not for me. Again; it's the way my brain works. Digital clocks since like age 11. Trust me, I put a lot of thought into it. When you're losing 1,000 feet per 6 seconds, it's important to chose your instrumentation wisely. :)
More to your point about the GPS, I'm generally not reading text. I'm looking at a 3D graphical representation of the road, and occasionally an arrow with a number indicating which direction I will turn at what distance. At least for me, it's pretty easy to perceive useful information from it relatively quickly.
As to your other point: you could slow down to the point where you can keep your eyes on the road ...
We're entering circular argument territory here, but I'll re-iterate: When you're legally compelled to operate your vehicle at or below a specific speed, you are obligated to be very aware of the actual speed at which you are operating it. Guesswork isn't acceptable at that point. That means keeping an eye on the speedometer. No, not staring at it. But consulting it periodically. (Apropos to the subject, I usually just glance over at the digital readout on my window-mounted GPS. For me, it's much safer and easier that way.) Personally, I'd far prefer to just be able to operate the vehicle at a reasonable speed and ignore the speedometer, and often that's exactly what I do. But when there's heavy enforcement, I'll comply with their demands, even if it means decreasing my safety margin a little bit by diverting my attention. *shrug*
The point being, there are all sorts of distractions related to operating the vehicle. Navigation (GPS-based or otherwise) is one of them. Dashboard gauges are another. It's just something that must be accepted as a part of driving.
Bottom line is that GPS helps make me safer. Risk trade-off, yes; it's a slight distraction. But it also helps me avoid having to do crazy "oh, crap, I need to turn left here" lane changes by giving me plenty of warning about where I need to be, it keeps me from having to look at a map or try to read my handwriting on a post-it note, and even gives me my vehicle's speed without having to look down at the dash. Relative risk.
I'm going to bed now. :)