You asked "Which ways? What features did you find desirable?".
I was impressed by the number of real-time controllers. It is easy to fatigue when listening to an arranger keyboard. The accompaniments are monotonous and the sounds are the same everytime you play them. Several arrangers allow you to layer sounds, but the layered sound can be monotonous too. However, if you can control the layer mix in real time, the sound is more interesting. You might also want to vary the volume of the accompaniment, as well as parts within the accompaniment. Having nine sliders and two foot pedals certainly facilitates this.
I do not know whether Casio's sliders and foot pedals transmit midi. I would hope they did, and so they would be useful for controlling an external sound module as well.
I use a Korg Wavestation with my PSR8000, synching the Wave sequences with the autoaccompaniment. I like the result. Unfortunately, not many synths have wavesequencing. However, I would like to explore combining an arpeggiator with autoaccompaniment to see if that makes the styles more interesting.
I did not play with graphic editing on the Casio, but I like the option of seeing waves graphically instead of just editing numbers.
Maybe another way to explain what I like about the Casio, is look at what I think will be the best arranger and see what it is missing. The PRS9000Pro will be the best arranger in my opinion when it arrives. If the early photos are at all accurate (obviously they got the keys wrong), it appears it will allow only one foot pedal, and has one pitch wheel and one modulation wheel. Even the Yamaha EX5 (the non-arranger synth with the same case as the PRO), had a second modulation wheet, a ribbon controller, and provided for two foot pedals. Real-time control is just too limited on this synth, especially when it gives you the options of adding VL or AN synths. These lacks are much more frustrating when you find the missing features on a "lowly" Casio.
To be even more specific, I can have three right hand layers or parts on my PSR8000. If I had two foot pedals, I could control the mix in two dimensions for a high degree of expressiveness. I will be able to do this soon because I am buying a separate foot controller, but this solution is kludgy. Casio provides the two foot pedal jacks on the arranger itself.
Hope this helps.