I remember when I did playtesting for a company along with 2 other guys. The developers were amazed that none of us used multi-touch, as they thought it could only be played that way and it was a big design consideration.
So yeah, get some people you don't/barely know to have a shot as often as possible. Half Brick (the Fruit Ninja/Jetpack Joyride guys) would go out each week or so and just hand the game to strangers on the street to get their impression of 2 minutes gameplay.
And polishing can go on until the end of time. To use Fruit Ninja again, it took 3 days to make the game itself and 3 months of polish.
I'd suggest going through and getting everything to the bare functional requirements (backgrounds scrolling cleanly, no clipping in movements, smooth layout transitions, etc.) and then when you have the whole picture in front of you, you can see what really needs the attention. Nothing worse than spending a week on something only to find it has made minimal impact and then discover one section of your menus looks like an abattoir floor.