After so many years I also have to move from doit.im. I have just payed one month ago for the PRO account for one more year but its not working and not reliable.
I made an extensive research and tested several other options.
For those of you that are also looking for alternative those 3 are pretty good:
TickTick has great functionalities both on PC and Mobile but is a bit hard to understand how it works. Once this is figured out you can use it in a way similar to doit.
Nirvana is probably the best in terms of following GTD but both PC and Android apps are lacking some major functionalities (like group by reference or quick add on the mobile).
Nozbe is also great and following GTD strictly but I can't get used to the interface.
The final decision for me was to go with TickTick, and I just transferred all my active tasks into it. Hope the above helps someone.
I'll miss you too DoIt! It's been five years already...but I need to move on!
Doit alternatives (I'll miss you DoIt!)