Not to be a stickler, but the map data comes from the server, not from the Maps app itself. Thus, even though you have the GM Maps app, you do not necessarily know whether the user experience will still suck on the final release day, tomorrow. They could push out some final massive improvement to the server data before then. :) (No, I have no special knowledge of this, and if I did I wouldn't say.)
Full disclosure: I am biased. I worked on Apple's maps team, last year, so naturally I want the app to be good.
Well, the reviewers have had access to the same data as I have and critized the map data quality - I doubt Apple would delay an update of their map data they have ready to launch just for the heck of it at the cost of a lot of negative commentary.
The app itself is great, unfortunately the data quality isn't anywhere close to even the quality the iOS 5 Google Maps data. Apart from the obvious flaws like no street view or transit data, the pure data is not good too unfortunately. Let's hope Apple improves it fast, but I'm not too hopeful considering that Google has spent the last 7 years gathering data and improving their maps (and getting a lot of exclusive info submitted by their users).
Full disclosure: I am biased. I worked on Apple's maps team, last year, so naturally I want the app to be good.