He's in the prime of his career, is solid against the run and has been a consistent performer. According to various Redskins blogs, he was one of their best defensive players last year. He's listed at 6'4" 287 which is decent (but not great) size for a 3-4 DE. As long as he's part of a rotation at DE, hopefully he won't wear down too much.
In terms of money, I'd be happy with 3 or 4 years at 4 or 5 per. But if he wants starter-type money (more than 6 per?), then I don't think he's worth it, because he's not good enough to be an every-down starting DE in the NFL.