It is a brilliant player moving to an enormous club at a great age, so it's hard not to get excited about the prospect, even as a neutral.
However, Madrid can be a tough nut to crack, even for the very best, and there is a lengthy list of flops that Mbappe will not want to emulate.
We'd might as well start with the most recent, and Eden Hazard has been such a big flop he appears to have inspired a whole new transfer policy at Real Madrid.
At the time it looked like a reasonable piece of business. The fee was high but the quality of the player couldn't be disputed.
It hasn't gone well, though.
Hazard has struggled badly with injuries and has been accused of being overweight by the media during his time in Madrid.
To make matters worse, he laughed and celebrated with former Chelsea team-mates on the Stamford Bridge pitch after they dumped Madrid out of the Champions League.
In three years, he is yet to make 20 appearances in a single La Liga season and is not even into double figures for goals.
No wonder the La Liga giants are desperate to get rid of him.
Nicolas Anelka was always a bit of an enigma. The Frenchman exploded onto the scene at Arsenal, scoring 17 goals in 35 Premier League games for the Gunners in 1998/99.
That form earned him a mega-money move to Madrid, although Arsenal's willingness to let him go should have been a red flag.
It took him five months to score a goal for his new club and at one point he refused to train, accusing the club of treating him "like a dog".
"Something does not turn around properly in his head," Real president Lorenzo Sanz said, and he was shipped off back to Paris Saint-Germain having scored just two goals in 19 La Liga appearances.
From: AC Milan
When Real Madrid signed Kaka there was genuine excitement around world football. The Brazilian had established himself as the finest player in the world at AC Milan and was the reigning Ballon d'Or holder.
He was also at the very peak of his powers at 27 years old, so it was always going to take a world record fee to land him.
Madrid paid it and pinned their hopes on him, but injuries robbed Kaka of the opportunity to justify the deal.
He won a La Liga title but said he "lost the joy of playing football" while playing for Real Madrid.
Nevertheless, Kaka still made 120 appearances for Los Blancos over a four-year period, scoring 29 goals.
When anyone ever mentions bad debuts, Jonathan Woodgate is rarely far from the discussion.
Woodgate did not even make a single appearance for Real Madrid in his first season at the club due to injury. When he eventually did play for the first time, he scored an own goal and was sent off.
Further injuries restricted him to just 14 games in two years for Madrid before they cut their losses and shipped him back to hometown club Middlesbrough.
In 2004, you'd have struggled to find a better and more respected centre-back in world soccer than Argentinian Walter Samuel.
He was so dominant in Roma's defence that the Italian press referred to him as 'the wall'.
However, he looked like anything but a wall at Madrid and was quickly shipped back to Italy and Inter Milan after one disastrous season.
No doubt much to Madrid's disgust, back in Italy he immediately became brilliant again.
Given how well he had done for him at Chelsea, there was no real surprise when Jose Mourinho reunited with Michael Essien.
The problem was that the Ghanian was only just back form a serious knee injury and he wasn't the same player.
Mourinho loved him personally as well as professionally, though, and a biography on the Portuguese coach claims he was furious when very few Real Madrid players turned up for Essien's birthday party.
The story goes that he told Essien his team-mates were selfish. Unsurprisingly, both Essien and Mourinho left at the end of the season with Essien having played 35 times for the club.
Ah that summer of Royston Drenthe. What a time to be alive that was. The explosive left-sided player looked a genuine star when he lit up the European Under-21 Championships in 2007.
He was part of the winning side and took home the Golden Player award.
Madrid came calling later in the summer to sign him from Feyenoord, but boy did things go very wrong very quickly.
The writing was on the wall when the Bernabeu crowd booed him in a match against Deportivo La Coruna. He claimed it left him with crippling anxiety and he promptly embarked on a downward career spiral that saw him play for the likes of Reading and Sheffield Wednesday.
In what would have been a fitting end to his football story, he retired at the age of 29 to focus on a rap career. He has since returned and currently plays for Racing Murcia, a Madrid-based club in the fourth tier of Spanish football.
Danilo wasn't necessarily bad at Real Madrid, he was just a disappointment.
The Brazilian full-back was decent at Madrid, but decent counts as a flop in Madrid, especially when you cost as much as he did.
The problem Danilo had was that defensively he always had an error in him, and he probably wasn't mentally strong enough to deal with it.
"An error [at Real Madrid], no matter how small, goes viral," he once lamented. "A mistake becomes very great even if it is small."
The Brazilian has since forged a good career at Manchester City and Juventus, but he has to go down as a Real Madrid flop.