My wife and I just recently obtained our Global Entry cards and our first trip will be flying into Amsterdam. From Amsterdam we fly on to London Heathrow. Will there be any really "practical" use or advantage of Global Entry when flying in to either of these 2 airports ?
No, there will be no real for use. Global Entry cards are really only useful for entering the United States.
Both the U.K. and Netherlands have programs that frequent travellers with Global Entry can enroll in at additional cost, but if you simply have global entry there is no advantage.
It won't do you any good entering other countries. It will get you into the TSA "Pre-Check" line when you fly from US airports, so you can keep your shoes and belt on. But it won't speed you through the equivalent security lines in non-US airports -- or, as we learned recently, through the security line to board a connecting domestic flight after arriving from overseas.
We joined the program a couple of years ago and it saves time when re-entering the US. We also got several e-mails, I think from Homeland Security, notifying us of several other countries' corresponding "trusted traveler" programs that we could join. I seem to recall Mexico and the UK in particular. We didn't pursue this since we don't travel often enough to any particular country to justify the cost and trouble.
Yes, Global Entry can save you tons of time upon your return home. On more than one occasion I have made it through immigration in less than 10 minutes when the regular line was ridiculously long. In my experience, the immigration line at SFO can be insane. TSA Pre is only available from certain US-based airlines so even when departing from the US, it may not be available.
I've got an Air Canada flight in June, LA--Toronto, and their website says I'll be able to use Precheck.
From the TSA's website see https://www.tsa.gov/tsa-precheck (apologies for the too broad statement above because Air Canada is clearly not a US-based airline):
The following airlines offer TSA Pre✓®:
Delta Air Lines