I will do a breakdown comparison based on what I have:
A. Software (User Interface)
1. PLEX - hands down the best. Users pay a subscription fee which in turn help staff software developers to innovate. I have a Lifetime Pass. The cons is the Transcoding. If you have a slow computer or are running multiple tasks in that computer, then you may experience buffering problems with your 1080p+ rips.
2. Kaleidescape - Previously it was the gold standard. But after years of neglect, I can't say that it's the best anymore. I have a very small catalog so it's fine for my current need.
3. Zappiti - User interface is rather clunky and seems very dated. It is serviceable.
B. Hardware
1. Kaleidescape - I can't tell you how much I love the build quality of K. Even though the K Strato-C costs 3-5 times more than the Zappiti, I would choose K over Zappiti. For $4k, it is priced reasonable for what you are getting. I know it is still high. But at a one time cost you get a very premium product.
2. Zappiti - I didn't go for the premium model (Zappiti Signature - $3k) and went for the Zappiti Neo for $1k. I like it! The build quality is very similar to what you would expect from a high-end Blu-ray player (without the optical drive). I couple this with a few Zappiti Mini across the rooms. The mini (at $250 and no longer supported) is very cheap in build. Instead of mini's, I would recommend buying a Neo for each room you want it in. I own two mini's so I decided to keep them until they break down. You can download the PLEX app in the Zappiti.
C. Eco-system
There's no need to do a comparison because only K has an ecosystem. The pro is that everything just work and you don't have to deal with the DIY hassle. The con is that if K goes out of business, then you have no options than to download all your movie content into your Terra Server or kiss your purchases good-bye. The switch-over cost is lower for the other two. If Zappiti goes out of business, then you can buy one of the other alternatives such as Zidoo. The only headache is to scan your library again and organize your movies from scratch. But your movie files are still in your NAS server.
D. Data Crash
If your hard-drive fails, you can re-download them with K. For the others, if you have a RAID system, you can replace one-disk at a time when one hard-drive fail. But if you are rebuilding your NAS volume and you experience a power-surge, you run the risk of losing everything! It happened to me so I have to make another back-up and store them somewhere safe. Cloud storage is just to expensive for me to make that commitment. Especially when you have over 200TB of storage.
E. Price -
1. Zappiti - It wins on this. If the unit breaks or if there's a newer unit, I probably wouldn't hesitate and replace it after I feel that I got enough usage from it.
2. Kaleidescape - K can actually overtake Zappiti on this. Since the newer Strato-C can download 4k movies in 10 minutes, there's no need to buy Terra storage. The only problem is that with the current UI, you cannot view all your purchases in your Strato-C. You have to download the movies into your Terra first in order to view your purchases. If K can solve this, then the value proposition increases. But right now we are hearing that K doesn't make money from movies but with hardware. But very few people are willing to spend $5k for a 6TB Terra or $25k for a 88TB Terra. So I don't really know how much hardware sales K is generating. If a 6TB internal hard drive costs $100-150, I wouldn't bat an eye and pay 3-4 times more for K's storage. But we don't really know the actual cost to make a Terra storage unit. So $1k for a 6TB storage may be their at-cost value and why they need to charge $5k as MSRP.