Very interesting discussion in this thread- usually it devolves in to two or three guys fighting over whether an HTPC can do what Kaleidescape does- and this discussion is not!
Personally, I am not quite financially ready to buy in to the BluRay setup in terms of server and disk vault for Kaleidescape yet, although I did pick up an M300 player for when I am ready from an extremely knowledgeable and well-connected forum member here.
I got in to K after admiring it for years by buying a used system at a steep discount. I had to get new drives but other than that, it works as it should. Previously, I had built several iterations of HTPC boxes and loved them... with hope that they would "break through" to K's usability. And By usability I do not refer to setup- I refer to overall ease of use.
However, I see the point made earlier about the experience being largely the same- once you sit down to watch a movie. You move through a menu, select a movie, and hit play. The only thing I will say to that is that K is intuitive in terms of its menu structure and disk contents selection beyond what else is out there, in my opinion. I find myself skipping around through different movies, watching a scene in one, and a scene in another and moving on to a third, and my wife can easily do the same. Other software I have tried has not nearly been as smooth for that kind of viewing
I love MyMovies and was active in the forum community for a long time as a moderator. I still check in from time to time. I think it is the closest thing to a K experience you can get on a non-K system. The problems that MyMovies faces in usability really are insurmountable when dealing with BluRay disks or new DVDs with advanced copy protection and player software that is buggy.
AnyDVDHD is required, and when new variations on copy protection are released, there is usually a delay in updating. Sometimes it is just a simple update, sometimes- as with the latest major revision- it took some time. There is also an issue with player software not properly playing a bluray, or not being able to always skip advertisements, etc.
That issue can be avoided if you rip to a single file that has no menu structure and maintain the same quality, but that requires custom settings for each disk.
The one thing I had pushed for in MyMovies but never got was a file structure component to the disk profile system, where people could input the names and content type of each file present on a BluRay and DVD disk, so you could automate the single-file ripping system in a hierarchal order of preference for audio tracks, lack of menu, etc. For example- MyMovies scans a BluRay and a user inputs the category of file each file is. Another user- using that profile, is ripping that disk based on the preferences to priori tie no menu, no advertisements, and Dolby TrueHD audio. The disk profile is read and identified by disk ID and the ripping software knows "how to" rip the disk properly to get just what the user wants. This would make ripping for all kinds of systems pretty trouble free for those users who do not want to copy the whole disk as an ISO.
XBMC has always been beautiful and quick, but I completely agree with Amirm's comments above. The selection of sources, etc. has never "just worked." I was reminded of the movie quote, "PC Load Letter- what the F$%& does that mean?!?" when trying to configure it. Eventually I got it setup, but it never just worked to allow me to add new files and automate cover art very well.
I think the closest you can get to K, right now, is the Dune Player + Windows Server combination someone mentioned above. That will get you the server+client streaming K does. Also, from what I have seen, the Dune Player Interface is constantly being improved and is approaching XBMC-like smoothness. Someone else can comment on this here.
I think the main thing that K gives is that extra 2-3% of usability above whatever the next-best option is. K is like a set of Meridian DSP8000s- they are incredibly expensive but they give you just a bit more range and detail than the models below them. So if you want the best media server system with the greatest chance of working all the time, even with newly released disks, you want the K. But you will pay for it- unless you can find a nice used system.