Mediocre engineering culture with severe lack of ownership
Pros
This is a tech review, so I'll focus on that. Booking's scale is big. So if you're into large amounts of data, interesting A/B testing, scalable architectures then Booking has a lot of potential. Be aware though that almost all of this is made a lot less interesting by the mediocre and half-baked approach to almost everything in technology. If you like getting paid for 40 hours but only want to work 32, then Booking is also great. Show up at 10:30? Leave at 17:30? Fine. Happens all the time. If you love Perl, then this is also a great company, because like in Perl there's more than one way to do it. So prepare to see every arcane Perl operator you ever knew, have three chat clients open (because there's more than one way to reach someone), prepare to work on lots of technologies that don't work together well and prepare to see many different management styles / ways to organise teams.
Cons
The engineering quality and technology is mediocre. A lot of teams are very business focussed (which is great in principle) in their particular domain but without anyone maintaining any kind of overview. This means that the default strategy is to just cram a feature in the existing code in the fastest way possible. That is, look for the piece of code that needs to change and add an "if (my_cool_feature_enabled) {} else {}" construct. Rinse and repeat until you have Perl functions that span 5000+ lines. No kidding. By that time everybody and their mother have touched that code, so no-one owns it anymore. Meanwhile of course some people have gotten frustrated with the spaghetti Perl code base that is impossible to navigate and is an archeological record of two decades of mostly uninspired hacks and copy pasting code (the latter happens to the extreme). So new technologies have been added. Which ones? Well, it's booking so no-one owns this process. So there's Java which is the only 'official' second language for that matter. But of course the data scientists use Python. And R. And there's Go, which is cool. And Kotlin, because it's also cool. Within the Java world most popular (web) frameworks are being used. And so are all protocols you can think of. Same on the frontend, React, Angular, it's all there in various places. This freedom is nice on the one hand, but it just doesn't work. It also leads to resentment between teams. "Why do they do their own when they could have used Spring boot", "why do they do this in Java when 3 lines of Perl would have been fine". Because no-one takes ownership or dares to make decisions, people do what suits them best. This is very pragmatic, but it leads to loads of islands of different standards and tensions between these islands. Standards not only in technology, but also in quality of people. Pointing this out invariably leads to an explanation along the following lines: "Well, yes but in order to understand this you need to know the context", and then a war story comes to defend the mess. And that's fair enough, but no-one cleans up after themselves.