After using one of these, you won't be bothering with tablet-only devices like the iPad2 or the GalaxyTab. The dual tablet and netbook nature of the Asus Transformer is unique and works VERY well indeed.
There are loads of expansion ports (MicroSD in the tablet, full SD in the dock, USB ports in the dock, MiniHDMI out and more).
It's really hard to fault this, it already has Android 3.1, and Android 3.2 is due any day now, Asus are very prompt getting the Android updates out.
The bundled office suite (Polaris Office) is very good, and of course you can use Google Docs, and there is a pretty reasonable selection of Android tablet-specific apps, plus you can use almost all the 300,000 standard Android smartphone apps too.
Easily a 5 star product.