The C#/G# mechanism is probably one of the most helpful key modifications. The C#/G" Alternation has been available on all professional German system clarinets for a century but missing on the Boehm system.
The C#/G" Alternation enables you to go from C# to B or G# to E without the smallest glitch. Usually as soon as you lift your pinky key from the C#/G#, the C/F key will open just half way, breaking up the airflow and sound of the note until the E/B key closes it again. Now you can leave the C#/G# pressed while operating the E/B key and get a perfectly smooth connection without needing to practice it for a second. This mechanism will also prevent the jumping/bouncing C#/G# irritating.