I have been a GPS user for a little over 5 years. I got a Garmin Street Pilot III for my motorcycle for my first GPS, and then last year as I prepared for a
cross country Cannonball Run in an antique truck, I bought a new Garmin. The SP3 had minimal memory and couldn't hold a lot of data compared to what's out there today. I bought a NUVI 650. I am STUNNED at how much information store this thing has and how great it is compared to the original SP3.
Since then I got my brother a NUVI 680 for Christmas last year. Its hard to believe that there can be so many improvements in just a short period of time but the differences are significant.
The NUVI 680 is an upgraded model from the 650 for sure. The 680 has blue tooth (handy if you have a blue tooth phone since the GPS now works as a HF phone setup AND it will auto dial waypoints, it has a subscription feature to avoid traffic delays and jams. That uses a MSN radio signal I think. The subscription is about $100/yr but if you live in a very congested area where traffic problems are common, it is DEFINITELY well worth the investment when coupled to something as powerful as the 680.
The information store in these GPS units today is overwhelming. Every street in the US is now in the NUVI high end models right out of the box. Easy to hook up to your computer through a USB connection, they can update with newest maps and waypoints.
How you use the GPS is a big aspect of which one to buy. If you are simply looking for telemetry such as a very sophisticated trip odometer, then the low end units for less than $200 are actually all pretty good. If you want blue tooth, and a list of restaurants with addresses and telephone numbers all organized by proximity to where you are right this minute, or movies with movie times, hotels, theater, attractions, and much more, then the higher end units will deliver large.
Decide how you will use the GPS and what features you want before buying then match your budget to your desires. If you have friends with portable GPS units, then talk to them.
The current max speed on my GPS is 724 MPH. I flipped it on while on a transcontinental flight a few months back when we were being pushed by 200 mph tailwinds just to see how fast over the ground we were going. I got stopped in my smart recently by a curious cop who looked at the GPS and his eyes bugged out of his head when saw the 704 MPH max speed. He asked if that was an error and I told him "no, but I was going down a long hill with a strong tailwind in the smart when I recorded that speed" and then laughed. He didn't think it was funny but I still do.
So to me, features are the driving force behind what you buy. List the features you would like to have and then go from there. It is amazing the places you will find within a few miles of your home that you never knew existed if you get one with pre-loaded points of interest such as restaurants, hotels, parks, etc.
They are EXTREMELY handy when you travel and frankly, I think a GPS is now as much a part of automobiling as a radio and heater are. Mine is either in my car or in my rucksack everytime I travel anywhere other than around town.
Good luck!