Infrared Tracking

I know that Russian fighters have this feature. The Mig29 and the various Su27 variants. I have a few questions about this feature:

1. How useful is it compared to radar?
2. What are its advantages?
3. Do US or other western fighters have this feature? If so, which ones?

Also, I read in a book that the Delta Dart had an ifrared tracking system. The sensor was located by the front of the canopy and was retractable. Does anyone have more information on this?