Press Relaeases are fine for both, building links and instant traffic (reach) to your site.
If you put some work into keyword (and tag) research, your PR will be picked up by many website all around the www, allmost all of them will link to your website or/and at least to your PR. This should help you with link building for sure!
If you asked if PR is enough in terms of SEO, surely not! You need to use various diffrent link building methods in order to make it look natural in the eyes of Google and Co.