WelcomeTaxi service was very useful.
I had a flight Oslo to Moscow-Sheremetyevo expected arrival 0:50, and had to fly from Moscow-Domodedovo 09:00, so with plenty of time for transfer with night marshrutki.
But the flight from Oslo was delayed 5 hours, compromissing the possibility to arrive in time at Domodedovo. I ordered WelcomeTaxi around midnight while waiting in Oslo airport, and despite the late hour, I got a very fast confirmation with name and phone number of a driver to wait me at arrival in Sheremetyevo. He was indeed there, provided me hints about online check-in for my next flight while we were on the way and in fact succeeded in driving me to Domodedovo with plenty of time ahead for check-in.
All in all, compared to tarifs provided with companies providing similar car transfer, WelcomeTaxi has been excellent. I paid 2600 rb for Sheremetyevo-Domodedovo, a price that one can't beat by bargaining directly with random drivers at arrivals, nor with concurrent services. The advice about traffic on the Moscow motorways, contact and follow-up were excellent all the time.
I will absolutely buy again from them if I end again in need of fast transfer between moscovite airports.