Acorn 3 last week (while it was still on sale) based on rave reviews from trusted sources. I used it to prep this image collage. I've been using Photoshop since the 1996, so this is a significant change.
Will it replace Photoshop? Maybe, someday. I'd like to be able to migrate away from Adobe, mainly because the software updates are expensive; I, a relative power user, really don't need many of Photoshop's capabilities; I find I need the other Adobe tools in the Creative Suite (web edition) less and less; I like supporting indie developers.
The problem is that my Photoshop workflow has evolved over many years. I can whip out images quite fast with the tool. Acorn appears to offer many of the tools I need (if not most, to be honest), but learning a new app and getting that speed back is going to take some time.
Learning to use Acorn efficiently feels akin to the time, years ago, when I learned to type in Dvorak instead over Qwerty. The above image would take me a minute to create in Photoshop. It took me 15 minutes in Acorn. But that's to be expected.
So far, it's doing the job well ... and it's fast, fast, fast. I also appreciate many of the little touches in Acorn that make it pleasant to use (e.g., when I add a guide, I'm shown the pixel measurement in a little bubble window as I scroll the guide into place). And I'd be remiss if I didn't mention that it supports all of my Dvorak-Qwerty keyboard shortcuts, which is something that I can't say for my version of Photoshop (CS3). So I'm sticking with it as my primary editor to see if I can make the switch. Even if it doesn't meet all my needs, I still have Photoshop CS3 to fall back on if I need more advanced features. My hope is that I won't need to upgrade to the newest CS version of Photoshop. Ever.