ikoon
As regards locking of the horizon … isn't it possible to script it somehow? The Z component of rotation should be zero, but simple “lock parameter” doesn't work. It is deeper somewhere, does it really require Sesi to implement it?
Yes it is something that needs to be implemented properly but the good news is that 3dConnexion provide a template that SESI can work from. Ideally we'll end up with the other camera modes too which help enrich the user experience.

Being able to change between target camera mode and camera mode is really useful too. Especially when setting up cameras in your scene (the camera mode is more of a first person fly through mode rather than rotating around an object).
I must admit I'm very disappointed by the 3dConnexion implementation in Houdini; it's as basic a level of support as you can get and it suffers from bad frame rates on occasions with no real explicable cause.
The main reason I stick with it is that I've set up my Radial Menus to be triggered by buttons on my SpaceMouse Pro. Really useful when modelling as I don't have to go near the keyboard.