In the early days I tried both KVS and expertflyer - they both provide similar information. It was a choice between them, going forward I did not perceive the need for both.
Being a techo, I really wanted to select KVS as the interface is good. However, the overall portability of E/F won out ... that and the 15% discount AFF Gold members get, which I apply to an annual subscription.
If you travel a lot and are concerned about the best fares etc, tools like KVS and E/F can be invaluable.
IMHO, two specific incidents taken just by themselves (let alone any other benefits), both relating to cancelled flights, easy searching for scarce alternatives & the subsequent empowerment of Agents to implement the solution, have justified the $500+ in subscription that I have spent over the last 10 years or so - but I do average around 80 segments per year.
If using for shorter periods, maybe just subscribe for around the times leading up to booking, and then flying, any major itinerary.