It is a good alternative to a netbook, that's why I haven't pulled the trigger on a netbook purchase!
We have iPhones, but it really does make a netbook a less attractive option when the Touch/iPhone are so easy to use.
If you have to type a lot, then use a regular computer or laptop for more rigorous tasks. Then enjoy the iPod Touch for anything on-the-go.
Twitter, Facebook, etc are all much easier for generic use on the iPhone. The Touch? well, since it doesn't have a camera, you miss out on the two-tap picture uploads. Hopefully the next gen Touch will have a Camera to make this possible.
Podcasting? the Touch/iPhone get it right. You are placed right into the store to download the podcast of your choice with just a few taps of the screen. Download & go. Listen to it in the background while you do other things.
As always, I suggest Pocket Tunes for anything related to internet streaming. Simply the best app around for this purpose.