The first few times someone tried that with me, it was repulsive, because they either had a bad-tasting mouth or their tongue was all mushy feeling. I had one boyfriend (I use that term loosely, since it implies an actual relationship that wasn't there), who had an excellent technique, though, and that was awesome!
I don't do that with my husband much. On our honeymoon, we were kissing like that, and all of a sudden he announced that his filling had come out. I didn't feel the filling, but the thought of it grossed me out so much, that I haven't ever been able to tongue-kiss without thinking of that.