So you had an Ultra? So did I.
Let's just put it this way, do you remember when v11 of the Ultra dropped and there was that "holy crap" moment. The AxeFXII was like that with v4 when I got it and the last two major firmware upgrades were the same kind of "oh my God" moments times about 3-10X depending on your excitability level.
Now to bring it back down to reality a bit there are some kinks here and there that I think that Cliff is working out, but I don't think that any of them are show stoppers; not even close. And really the biggest issues are AxeEdit right now and thats being worked on. I just think that its important to note them if we're going to dog the KPA out for reliability or performance. You have to be fair or its not a very honest evaluation.
Here's the way that I would evaluate this, the AxeFXII has a list of features that the KPA doesn't offer. Don't get into subjective crap with identifying cold hard facts like USB, PC editor, built in sound card, etc. Identify the features that you have to have and see which one covers everything you need. If the KPA can do everything you need to then move onto the sales pitch/subjective portion of this thread.
I like the KPA even though I haven't touched one in person. I don't feel threatened by stuff that I don't own and I don't feel compelled to buy every new product that is released for the sake of buying it. There are definitely some things that the KPA brought to the table in terms of approach that are very worthy of respect. With that said I think that with any new product that the hype machine gets rolling and there is a tendency to emphasize the positive, ignore the negative and that just doesn't help anyone who's trying to make an informed decision. I think that the last month or so has gotten enough of these things out there to get a bit more of an honest assessment. Its a great product, does what its supposed to do very well, but there are some trade-offs and like any new technology some growing pains involved with it.
For me the amp modeling improvements between the Ultra and II were enough to make me choose it over the KPA and keep going. I get the hands down best amp modeling system on the planet and I can create my own patches from scratch. When I want to tweak or create the knobs will respond and react closer to a real amp because the code is based on the actual circuits where as with a profiling system I can adjust a few things about a sound, but the pots don't respond like a real amp do. Secondly if I want to change a cabinet or mic type I can where as the KPA has it baked into the profile. The effects have been beaten to death; it's not really a fair point of comparison.
Even with all of that based on just traditional amp modeling vs profiling I can see arguments for one over the other. The TMA block just tipped the scales completely IMHO. Personally I never loved the idea of profiling for some of the reasons listed above, but mainly it was because I just don't have any of the amps I want to hear and the idea of mic'ing up actual analog versions of something to make a copy of it just seems kind of redundant. I know that if I had a KPA that I could probably dig through all the profiles out there and find some stuff I liked, but as the end of the day I'm stuck with using other people's sounds. If I want to tweak one of those profiles there is a limit to how far I can go as well. But even if I did go out and buy an amp now I can make a match of it now. More importantly though I can find a clip of a guitar I like and match it. That's something that I can actually do. Even cooler than that though is that when I go into tweak the tone pots and gain and stuff it reacts like that amp would if it was in front of me. Perhaps not exactly, but closer than anything out there.
If price is the sole determining factor then it is what it is. The KPA is an amazing piece of gear and I don't think that anyone can argue that. But if you are looking for what meets your needs the best and the price difference isn't a factor I honestly think that its really hard to pick the KPA over the II. Neither are absolutely perfect at every single thing they do, but both are really damn good at it. Workflow and feature set are really biggest separating factors.