I wouldn't call Tarragona one of the most significant WHS, but is a lovely place to visit. The sites are all in good condition, although the ampitheatre needs some rather unsightly and unnecessary barriers removed, and the forum needs a little tidying. The entrance fees are very reasonable, and you won't be over-run with crowds. The forum was a particularly rewarding experience - I was the only person there!
If you're in the area they are well worth a visit.