So far used Uber in Paris and Budapest.

For Paris, I only used them for travel within the city as their pricing to/from CDG is higher than LeCab (who offers a good service for their price). Positives experiences with all the Uber drivers. Very friendly folks who would get out and open the door, including those who drive for UberX. I was lucky and got a Mercedes E Series for one of the UberX rides too!

For Budapest, I didn't use them from the airport as I didn't hear much about Uber in Budapest. The social media pages for them aren't very active, so I just took the taxi from the airport. However, I did use Uber a few times within the city and have to say it was an average experience. Half of the drivers were driving pretty fast so it was a bit uncomfortable for my liking. It's worth noting that only UberX is offered and the cars are all pretty small, so if you have any luggage you might be better off with a taxi or public transport (if this works for you).