This is an interesting question, because I am of the opinion that Shakespeare is very careful to present Shylock in either way alone, but that he works very hard to maintain a tension between these two presentations of Shylock and our response to him. Certainly there is enough evidence to make us feel revulsion when we consider his hunger for revenge and the way that this consumes him. His desire to claim his "pound of flesh" to the exclusion of all else should make us sit up in disgust. However, having said this, we must remember that there is equal evidence that could be used to suggest Shylock is a character "more sinn'd against than sinning." Consider the way that he has been treated by Antonio and then how cruelly his daughter's defection and robbery of his riches, including the ring belonging to his dead wife, impacts him. In addition, we could also say that Antonio's "mercy" of sparing Shylock's life but insisting he convert is actually the final act of meanness that should make us feel pity for Shylock.
In a sense therefore, Shylock is a character that excites equal amounts of pity and revulsion. You might find it interesting to consider the way in which various productions of this play cast Shylock. For example, in the recentish Al Pacino version, Shylock is cast as a sympathetic figure, whereas other productions (normally older ones) invariably cast Shylock as a diabolical figure.