Responding to "it's a waste of my time to write the same email a thousand times" with "you should reply to each email personally" is moving the goalpost. The OP is about sending a first welcome email to every user that signs up, not about replying to an email sent for customer service or technical support.
I would never reply to a client using a canned response, I hate those with a passion. But the first email being sent, the client didn't send anything, doesn't know you or your writing style, didn't offer you any chance of customizing because it's just a "welcome to our service, thanks for signing up, I'm there if you need me" email. My point is, write it as if you would send it personally, but automate the actual sending.