The LCD is a bit grainy on the a540, but it doesn't hinder the use of the camera.
The a540 has two features that I consider a significant advantage: AA batteries, and a bit more zoom (topping at 140mm vs 105 mm 35-equivalent).
The other gripe I have about the a540, after owning it for about 3 months, is that the flash recharge takes several seconds. There are plenty of times when you'll be waiting on it to be ready for the next shot. Don't know if the SD600 is better at that or not.
Haven't found the perfect pocket digicam yet. I'm looking for one with all the versatility of my T-90, using one AA battery, with a flash that reaches to 60 feet and recharges before the previous shot is cleared from the screen. And pocketable, of course!
I crack myself up!