I came for Happy Hour on a friday and the place was pretty busy, especially considering Tivoli is a Happy Hour heaven with every restaurant in the plaza offering great deals. I had 2 very strong margaritas for about $5 each. I don't think they were the best tasting margaritas in town, but they were pretty strong. Just like any other Mexican restaurant you get the complimentary chips and salsa. Their salsa had a nice kick and their Happy Hour tacos were tasty, though a little small.
Most Mexican restaurants taste relatively the same to me and their menus are usually the same fajitas, tacos, etc. type of selection. As far as a great restaurant, I don't think Cantina is a must try. It's a nice Mexican variety but it won't blow you away. The Happy Hour is quality though so come 4-7pm if you want a strong margarita.