Stable like a swiss watch. Have Testing on an old computer and in a vbox. Neither with any problems.
To prepare for Bullseye, I decided to have a go with Testing on an additional SSD on my desktop machine.
End of December first tests with alpha 3 - after installation (no faults), reboot, grub screen and then a black screen. No terminal, nothing.
Contacted the development team and learned, that there seemed to be an issue with the 5er kernel and the loading of add. firmware during installation.
Did another test last weekend with the daily build of Testing. Same situation, grub and then ... nothing.
So to crosscheck I did an Buster installation in efi mode on that add. SSD to exclude any faults on my side. Buster with the 4.19 kernel - no problem.
A bit frustrated about the situation, today I removed the Nvidia 1030 and replaced it wit an old ATI X1550 and ... Testing booted

Added non-free and contrib in the sources and installed the nvidia meta package. Shutdown, exchanged ATI with Nvidia and no issue any more.
What surprises me is, that I don't read anything about that issue. And I am not with some exotic hardware here ....
br KH