I've been to Catina four times now. Once for drinks and three times for dinner. I've never been when it was too busy, so I can't comment on the seating problems...I've always managed to get a table within a few minutes of arriving.
First, as a bar, this place is great (4 stars). The scene is relaxed, the outdoor space is plentiful, and the drinks are tasty. No complaints at all.
However, as a Mexican restaurant it's boring. The food is bland and unoriginal. The tacos are too thin and so they give you two with each order, which just makes your taco taste like shell and masks the flavor of all the other ingredients. The corn was way over cooked (and over burned) and was rather tasteless. This very well may be the best Mexican restaurant in Pittsburgh (though I disagree), that's not saying a whole lot.
Pass on this one unless you're just out for drinks (and guacamole...which is actually pretty good).