KFlop support is excellent. There's a yahoo group, and a forum on cnczone, where Tom Kerekes will often respond (often within hours!) to any questions you may have. The only drawback to the KFlop is that you have to do a little bit C coding to get it to do exactly what you'd like it to, however they provide plenty examples, and Tom will help you with anything you're not sure about.
If you do go the KFlop route, I'd also suggest adding a Konnect, as you quickly run out of digital IO just relying on the Kanalog (it only has 8 digital INs and 8 digital outs, which isn't that much by the time you start counting all the critical IO that needs to be handled directly by the controller).
CS-Lab stuff is good, however I'm not sure how good their support is. It's all configured through Mach, so it's not quite as versatile as KFlop, but it's perfectly capable for most normal Mach controlled machines. Hood will most likely be along shortly singing it's praises and filling in the details!
I can't comment on Vital, as it's been a long time since I even briefly looked at them.
One thing to bear in mind, is if you do use Pokeys with other controllers, is there will be a delay between pressing a button, and the controller reacting. Mach3 only has a 10Hz update rate, so there is potentially a 200ms delay before the controller gets updated to handle the button press, then the controller itself may take time to respond to the update.
However, this shouldn't be a problem in Mach 4 as it has a far higher refresh rate.