RMAN Tablespace Point in Time Recovery or TSPITR

One detail that seems to get over looked when performing TSPITR on a "large" database is that the auxiliary instance is a duplicate of the all UNDO tablespaces, the SYSTEM tablespace and the TEMP tablespace.  So if these are gigabytes in size, your auxiliary database will need lots of available diskspace and not to mention the hours it will take to build these structures.