the new subarus are "famous" for having to change the entire engine after "X" kilometer, but apparently there are no major reliability problems in the 2011 impreza, But, here are the drawbacks. At 150k+ kilometer you will need a new timing belt. It will cost you a bucket load of money. Up to €1000. Also, subbys need a lot of service, especially the brakes. And, many of them have head gasket problems around 150k+ kilometers.
So, be very careful, and have the car inspected by a good mechanic before buying.
btw a "reliable" new subaru is the subaru brz