I have to agree with Andrew_Kerr that this site deserves higher ratings than it was previously receiving, and from reading the older reviews it seems to be largely due to the way in which the tour system has been re-worked. Whereas previously an in-person tour was required, the availablity of these has now become significantly more limited as they have been supplanted with an audio guide that you can use to enter the full monastery at any time and tour at your own pace. (I say "full monastery" to refer to the large section open to tourists, though, as this is still an actively used monastery so there are sections exclusively reserved for the monks which travellers cannot enter.)

Poblet is located decently far away from any major cities, so the tourist traffic here is significantly less than you'll find in other WHS sites. I've been a little bit over-exposed to monasteries & cathedrals during my past nine months living in Spain so you should expect that if you haven't been seeing a new cathedral every few weeks, this site will impress you even more. The audio guide is well narrated in multiple languages and helpfully tours you through each section and room of the monastery, ensuring you don't miss anything. The stark Cisterian style of architecture with limited decoration is quite powerful and lends to a sense of austerity in the place, but not so much as to make the location boring. It's neat to see all the various rooms where monks over the centuries have carried out their different activities, like cooking, studying, worshipping, etc. Poblet is worth a visit if you have the time and resources to make your way all the way out.