Yes, it's a superb extension, made my life so much easier as I go out to Rutherglen and East End a fair bit from Partick so means right thru the tunnel and onto the M74, brilliant.
As you say it's a pity they have throttled the speed limits on it. It's never made much sense to me why you can go out on single track country road and be doing 60 yet travel on a 3 lane motorway and be limited to 50.....