I enjoy it less, but never discard it. It's still perfectly fine tea, just cold.
I enjoy a warming cup but I don't like to burn my tongue, so Sencha is perfect for me in more ways than one
Also, with some teas, by the time they are cooled down, all of the "sediment" will have settled all the way on the bottom of the cup, which makes the liquid on top taste different. I'm sure some may even prefer that in addition to the cooler temperature but I like the full taste and usually shake my cup a little to mix it all up again.