Out of interest, I have installed Tempomaster as well as SpeedAlert and have been running them at the same time to compare.
Around where I live (rural Devon in UK) the fallback speedlimit that SpeedAlert uses gives it a huge advantage over Tempomaster. On a quick 5 or 6 mile round trip locally, I would say more than 50% of the time Tempomaster just displayed a ? for the speed limit, where SpeedAlert showed the correct national limit for the road type.
I also prefer that SpeedAlert changes the colour of the actual speed limit if you go over it, with Tempmaster it is the speed itself that changes colour and I prefer not to display that as it is already displayed on the main dashboard. Doing that with Tempomaster would mean no visual alert.
The only thing I did prefer with Tempomaster was the actual display of the speed limit. It does allow a much larger display, and the text is slightly thinner which to my old eyes was clearer to see. It also has a nice silver bezel effect around it.
But overall, for my use case and where I live, I will be sticking with SpeedAlert.