I got this card this weekend and I'm really liking it in my FG rainbow deck. I took out Improved Antimatter and put this nymph in, and it's been nothing but awesome when I draw it. (well, it was lame against Decay but it was awesome against other decks :p) I never thought of antimattering itself to avoid dying to shields, but it would have to survive a turn first (unquinted) against fire shield/spine carapace, but upgraded it can survive one fireshield hit and, if lucky, not get poisoned the first attack by the carapace.
It does look rather funny as the only unupgraded card in that deck, but I figured I'd keep it unupgraded for now in case I want to play in tournaments, which are almost always unupgraded only. (though I haven't been able to since I started work again) Plus, gaining only +1 toughness on the upgrade doesn't really do anything in most cases, other than buying that one turn against fireshield or surviving thunderstorm, which nobody uses anyway.
Of the nymphs I have so far, this is by far the best one. (2 fire, 1 aether, 1 water, and this) I still think the Gravity one is the best, but this one and the time nymph are solid runners-up. Good news if you like this nymph, though; I heard there's a bug that means you are twice as likely to get this nymph as a different kind, but I can't confirm that, I just read it in another thread once.