There are always issues when you are travelling to a foreign country where you don't speak the language, huge issues that can generate a lot of problems, and this is why I always tend to hire a private transfer company to drive me towards my final point (which can be a hotel, a conference hall, or the place where I actually need to go - it depends a lot on each foreign visit).
Unfortunately, while traveling to Bucharest I had huge issues in finding an international company (like Sixt, or even a cheap one, like Enterprise) as all the companies had ALL their cars with drivers fully booked - so it seems that there are a lot of things happening in the city of Bucharest, which to be honest is one of the biggest cities in the eastern Europe and the capital of Romania, a country inhabited by about 20 million souls.
So... I was in a huge difficulty trying to find a solution for my local transportation, as all the companies that I know were already fully booked, so I started searching on google some more offers )(after searching again on Expedia, but without any kind of luck).
Finally... after many tries I have found a local company which operates only on luxury cars (usually Mercedes) which is great and they provided the transport from Otopeni airport services at a great cost, in excellent conditions an of course, with a private driver.
Finally... I can say that I used the local company also for transfers and transportation in other places in Romania, which has really been great andd this is why I am recommending those here, as they provide really good value and quality.