I have been using OpenHAB and MQTT for a number of years now.
The reason for using MQTT and not just a serial connection from the MySensors GW is partly the flexibility as many have stated previously here. Easy to test, using e.g. the mqtt.fx tool. If you want to know what is going on you simply connect to the MQTT stream and "snoop" on the messages.
Another reason is that I am running three geographical sites on one OH installation. OH is running on a NUC at my home. There is also an MQTT GW here. Two other My Sensors MQTT-GW are running remotely in two summer houses. The three GW connect over internet to a cloud-based MQTT broker and OH connects to the same broker. In that way I do not have to open any ports into my home network for access should I have had a local MQTT-broker at home.
There doesn't seem to be a ton of interest in this but I wanted to provide an update. After writing the transport and using it for a bit I've decided to go another direction with the integration to SmartThings. I felt like the custom transport would require more maintenance and I wanted more of any option that was configuration rather than coding. Because there was C++ code on the MySensors side of things and then Groovy on the SmartThings side it just seemed like a brittle integration with the design and how I wrote the code. SmartThings is going to be moving in another development platform direction as well at some point.
I'm working on an MQTT solution with Mosquitto and Node Red (on a Raspberry Pi) and it will also require some Groovy on SmartThings but a much simpler solution IMO. There will be more to come and I'll share here when it's complete but I do have a prototype/proof of concept working. The two way communication is much better/cleaner IMO than how I did it with the transport. If this is something that interests you let me know and I'd be happy to collaborate.
@monte Sorry for the late response, I was working on other HA integrations so I left MySensors for a while.
But now I had a Domoticz jam for the third time, so it was a trigger to make the move with at least one gateway.
I did indeed see some values in HA, but, as you may have seen in the other post, I have been so cheeky to just modify the JSON file, and that works!
I will, however, adapt new sensor nodes to the format desired by HA.
By the way, I want to make you very happy with HA now, what a lot of integrations are possible!
And it looks even better than Domoticz!