It's a whole new world.
Seriously, everything is better. The OS is more stable. There are more apps, most of the interfaces are cleaner and better-looking. The apps I had sport new, better functionality. Even the battery life is improved. The only downside (and this is a pretty minimal downside) is that it takes about twice as long to boot up.
I spent a good long while on Saturday downloading free apps: OpenTable, Google Sky, Twidroid (which is my favorite of all the Twitter clients I've tried), they even have that stupid light-saber app.
Why they can't factory load these with 1.6 (or even 1.5) rather than push out over-the-air updates to new phones is a mystery to me. I mean, I bought this thing from T-Mobile--they were just going to have to update it anyway.
Still, happy with the improvements.