I have two visionariums myself. I picked them up at a show in late January.
Pros:
Can put 2 snakes in one viv - there is a cage seperator you can snap in
I like the sliding glass doors, look good aesthetically
The plastic pan makes them easy to clean
Cons:
Humidity is very high in the lower vivs I have found.
Aspen gets stuck in the cracks where the doors slide
Seems to have poor ventilation, especially the cages on the bottom
With the cage divider in, it makes the inner door hard to slide
I would think that these cages are good for adults, as they are the same size as a 20L. But remember, thats just a MINIMUM enclosure size for an adult. I think that they would be good for smaller adult corns and yearlings. Honestly, I should have put my money into a regular 30L for the same, if not cheaper, price. What makes them nice, is the stackable feature, which is why I bought them. But because of the high humidity in the lower cage, I was worried about the aspen molding.
Here is my current set-up. I dont even stack them, but they still look nice. I have 3 corns and one cali king in the visionariums, and a corn in the 10g. I think I will end up using one of the visionariums for my king when he gets bigger, and put my smallest corn in the other.
As for using a UTH, thats what I use. I have one under each visionarium in the middle, so that each snake gets their belly heat. They work fine under them, but just like aquariums, I wouldnt stick them directly to the bottom.
Hope this helped!