I highly doubt most players won't be able to tell that it's a synthetic voice after only a few lines, but it's impressive none-the-less. I probably wouldn't mind it, as an extra option over nothing. Personally I have a fairly low tolerance for bad-to-middling voice acting and I'd probably prefer a robot over an untrained human. Nothing breaks the immersion faster than when you hear the actor, not the character, reading their lines and doing their darndest to sound authentic.
As an aside, I recently tried implementing text-to-speech in my own game (just using the standard Windows API) and while it was really funny to hear a robot say words like 'pussy' and 'cock', the novelty soon wore off. Even if I made it an optional feature I doubt many players would leave it on. Also, I had to do some extra work to get it to properly pronounce words like 'cum' and 'fellatio'. Sometimes you have to do a lot of massaging to get a word to sound right, and the less of that the better.
