Is this vehicle generally reliable?

When they are new and low mileage, they are not that bad. However, as they accrue mileage though they start to show cracks. Pretty much they are a GMC Yukon Denali with Cadillac badging. The transmission starts to fade or fails because it is lighter duty for the size of the vehicle.

I heard the mileage isn't bad and the engines and transmissions are reliable?

Also a 2nd opinion wouldn't hurt!

Yes, these are well built, as Steve says, and this is one of my favorite vehicles. If it does break it can be fixed much easier than newer cars. If it is in good shape check the condition of the transmission fluid and the engine oil. This can tell how the vehicle was serviced. Upload images of the vehicle if you need more info.