I have just managed to get Cortana to work on my Lumia 550.
The only thing I really need it for it to handle texts while driving, this I will test out over the next few days.
I've tried using it more frequently, but the fact that she never hears me when I say "hey Cortana" really kills it for me. I've played with the settings for that - just my voice, anyone's voice, additional recognition tests, etc. Nothing really helps it. And then it will start listening out of nowhere when I don't need it. Definitely a thumbs down from me. BUT if Microsoft improves it, I am willing to give it another shot.
As far as I am aware, Cortana is still the best digital assistant for hands free SMS over Bluetooth... Use it daily on my commute to and from jail (work).