If you're doing any professional web work you will need Photoshop and Illustrator at a minimum. There are cheaper/alternative versions out there, but you never get 100% compatibility. I have Pixelmator on my home machines, and prefer to use it for small bits of work, but at work when I receive designs from colleagues, they're always in Photoshop formats. Pixelmator consistently has color-matching issues, and the "Elements" versions from Adobe don't open layer groups, which makes them unusable for all but the simplest designs.