I use both KVS and ExpertFlyer. I like them both but for different reasons.

On advantage to EF is that it's web-based so I can access it more or less anywhere. KVS needs to be installed and it requires a rather specific unlock code. So if my machine crashes or if I upgrade to newer machine, I need to email KVS and get a new code. I find this cumbersome, but KVS is really good at getting back in a timely manner and is quite accommodating of requests. KVS is also a Windows-only app so I need to fire up Parallels on my Mac first before I can use it. This is not a big deal, but it is an extra step.

Having said all that, I like some of the features of KVS and tend to use it as my primary tool.