Well, there's evidence to both sides of the tale. As we know it, and we're perceive Nessie to be...which is a water-dwelling unknown creature, whose relys on the lake for food, with no other possible creatures living with it, and no other way out of the lake, then there's probably very little possibility.. especially considering that they did do the sonar test.
However, there are possibilities that we may not have accounted for. There's just as much "proof" or support to one side as there is to another. I think basically on this one, you just sort of have to go with the side you want to believe. Personally, I figure it's probably not there, but I like to think that in the same manner that fairies, dragons, unicorns, sprites might just, by golly, show up one day and there will be magic and mythos in the world, that darnit, Nessie might just be there too. And I'm just going to enjoy that thought as long as I can.
Now, if all that happens *and* I win the lottery... I'd be happy as a bug in a rug (or Nessie in Loch Ness).