We visited twice, the second time to attend mass on Sunday.
On Sunday morning there are volunteer "guards" posted at the gate who don't like tourists to go to the mass and then leave early but we persuaded them we would stay for the entire service, which we did. The church is very historical and amazingly ornate with significant spanish/moorish influence in the geometric decor.
During the day you can visit the sacristy and also the bells which were saved from the collapsed campanile after an earlier earthquake. This is a not-to-be-missed church in Quito.