I've seen this soda, and never tried it. I always say "Ewwwwwwwwwwwwwh" when I see it.
Funny, I was just at Rein's, a wonderful deli just outside of Hartford, CT, this past weekend, and they had Cel-Ray soda (not to mention killer pastrami). Dr. Brown's also makes root beer, black cherry, and cream soda. Here's what Wiki has to say:
"Dr Brown's Cel-Ray soda is a soft drink with a celery flavor. It is fairly easy to find in New York City. Outside the New York City region, it is rather obscure but can sometimes be found at Jewish delicatessens and restaurants. In addition, it can be found at certain grocers that specialize in American food in Israel, and other specialty grocers.
The flavor is reminiscent of ginger ale, but with a more pronounced celery flavor that is more pungent or peppery, derived from celery seed extract."