I use PaintToolSAI. Photoshop has always gotten on my nerves, and every one I've used my pressure sensitivity on my stylus never shows well, which I REALLY hate. SAI's pressure sensitivity compatibility is amazing and definitely 20x better than Photoshop's. Photoshop is also absurdly priced at an f-ing $800 for one of the full programs, whereas SAI is $100 and you can get the full program for that price. With Photoshop, unless you're at a business where you're forced to use it(e.g., the employer requires it or a class in school requires it(which, in most cases, they'll have on their own computers for you to use)), you're just eating the cost for something that you really won't need. SAI offers pretty much all of the same stuff and, what it doesn't offer in the click of a button, can be with a little more effort.
I have both. Bought and paid for, as I'm a photographer with my own company (just a small time company, but I make some nice money).
For photography, interface and layout purposes, nothing beats Photoshops workflow.
But for drawing - well, having used Photoshop since the very old v5.5 (not CS5.5 which is the latest, but the v5.5 before they made up the CS brand), I'm a huge fan of Photoshop. But the first time I tried Paint Tool SAI - I immediately paid the few bucks that it cost.
Now, I only draw in SAI. Check out my gallery - there's a few drawings from photoshop (the crouching salsa girl) and the newer drawings are made in SAI.
Download the demos for Photoshop and SAI if you want - I'm sure you'll reach the same conclusion as I did...