I've only bought a few Android applications, but I've not been disappointed.
I love the new Swiftkey Keyboard. Never seen a keyboard predict text so well.
Titatium Backup is awesome, the free version works, but can become tedious since it doesn't automate the restore process.
I would pay for SetCPU, but being a member over at xda-developers.com, I got it for free!
Winds of Steel is still one of my favorite games.
Minisquadron has a very cool interface and is lots of fun.
When Angry Birds comes out with a full version, I'll probably buy it too!