I don't own this stock because it's always seemed to be too...

I don't own this stock because it's always seemed to be too expensive, although I'm not convinced about that.
I follow it closely and have listened to a few interviews with Sam Hupert over time and made notes as best I could, so I offer the following in response to your query,
(in accordance with my notes)

Pro Medicus offers a software suite that resides on a radiologists' desktop. This software provides a full range of detail which they refer to as informatics.
There are about 40 to 50 competitors with Pro Medicus being about the fourth or fifth largest supplier as of a few months ago.
They believe that they have between 5% and 10% of the US market at the moment.
But their offering is unique.
All other providers of the service use a totally different technology.
These providers take large imaging files, compress them and send them down the network to a fairly heavily configured work station where it would be unpacked with the processing then done locally involving skill sets and time.
Pro Medicus, as best I can understand it, perform the processing work up front, in the cloud I think, and stream a completed process direct to the radiologists' desktop thereby providing a huge time saving. The "simplified" result enables a radiologist to utilize the offering pretty much straight away rather than passing through a learning curve as they build the end result on their desk tops.
The technology employed by Pro Medicus has taken a long time to develop and he said that he's not aware of any other organization that is attempting to achieve what they offer,
They currently support radiology tests but are looking to envelop skills with cardiology tests as well, as the requirements (apparently) are not too different.
The contracts they've won over time range in terms of periods from 5 years to 10 years. Some of the early contracts have expired and every such customer have renewed their contract.
The business is efficiently run from a cost point of view. He said that the first customer (a small partnership in Melbourne) works with exactly the same software package as is used at the large US hospitals that bought in just recently. There is one version of the software only and that is implemented seamlessly which makes for a cost efficient process, hence the pleasing look of the financials that are presented each 6 months.
They're financially conservative with little or no debt on their books ever and no need to raise further capital.

So, to me, there's not much question that this is a great business. It's all about the buy in price.