So - despite the above - I ended up buying it.
I still won't be using it as my primary nav app. However - if you have a Samsung phone (I have a Galaxy Alpha) it will run in multi-window.
That means I'll be running it alongside Magic Earth Pro or Google Maps (both also work with multi-window) as an alerts app. Just plan the route, set voice to alerts only (without directions) and it can provide alerts for speeding, speed cameras, traffic on route, faster routes detected, tight bends and level crossings - with all set to whatever text to speech phrase or random sounds you might fancy.
It clearly displays the speed limit, and a route bar at the base with traffic incidents indicated as red or yellow bars for the full journey. Upcoming speed limit changes are shown very clearly.
I just wish you could turn off ETA. Ran it today on my commute with Magic Earth Pro. It's about a 10 mile journey - and it predicted 5 minutes faster than Magic Earth Pro. I arrived within a minute of the predicted Magic Earth Pro time.
Running both together - it was clear that Magic Earth Pro had realistic turn time penalties at junctions and Sygic didn't . Sygic has suggested a couple of detours around traffic onto minor roads that Magic Earth Pro didnt' - but given the lack of turn penalties I doubt they would save much time. Magic Earth Pro has managed one more significant detour around traffic - a longer route on main roads - that Sygic missed.
If you don't want to run two sets of nav apps on screen you can adjust the multi-window sizes - or Sygic can be set to run in HUD mode and it will just show the speed limit, your current speed, the next turn and an ETA - on a black background.
If anyone has been wondering how I've informed my opinions of these Nav apps over the last few years - that's it. Head to head testing at the same time, on the same route, usually on the same phone. Google Maps, Magic Earth Pro (and Magic Earth, which uses Openstreetmap istead of Tomtom maps but does still use Tomtom traffic data) and Sygic are the only ones I've found that will run in Samsung multi-window though. Other testing (with Tomtom Go, Waze, Navigon, Co-Pilot etc) has been done with one app in the foreground and the other set to voice directions and alerts in the background.