Is Uber popular in Amsterdam? Or is there another taxi app I should use while i'm there?
If you MUST take a taxi (try to avoid it, try to use public transport or walk or bike), use Uber. Uberpop is actually illegal. It still exists, but there are some incidentens lately that those cars get stopped by regular taxi drivers and the uberpopdriver gets beat up.
The times that I had to take a regilar taxi I had some real asshole drivers. They try to charge you for your luggage or make a big detour. Only had good experiences with Uber.
Taxi's are relatively expensive so only take a taxi if you must. For example, for a ride to the aiport from east part of town you pay €50 for an Uber, regular taxi can be a bit cheaper.
That's simple: UBER! (or Uber pop!) It's perfect, way cheaper than regular taxi's here. Plus your driver is nice and does not phone/text AND drive at the same time, since he will get a review. The average 'normal or regular' taxi's take detours and don't care what you think since there is no review system. (Yes, there exist some good exceptions in the regular bizz but on average: 6 out of 10 and with Uber ten out of ten) I LOOOOOOVE UBER and consider it my only taxi service here, there is another taxi app service called Yeller but that is still from Taxi Centrale Amsterdam that sends out the 'regular' taxis.
Uber is very popular in Amsterdam, so you should stick with it.
Taxis are expensive and for trips within Amsterdam, you won't pay much more for an Uber. Taxis in Amsterdam are pretty terrible. The drivers would always try to rip me off, even though I speak Dutch and was not a tourist (not having change for EUR 20, driving out of the way, not turning on the meter, etc.). Once Uber arrived I always used them instead of a taxi.
If you use a Taxi try to find/call a TCA driver (they have an app I believe), their company holds them accountable from my experience. Do not take a taxi from the Central Station or Leidseplein - you are almost guaranteed to have a bad experience.
Sabine makes a good point - generally within Amsterdam public transport is very good and can be faster than a taxi for some routes.