With all their stars back, the Blues could do well. But untill they sort out the problems they have at 3 and 10, they'll loose the close matches against top opposition imo. Henson could be an answer at 10, possibly...
The Ospreys will have a decent chance as always. Their home record in Europe is second to none (6 years since they lost at the Liberty), winning the home games always gives every team a chance of making the quarters. But after that, I think they're lacking in a couple of positions to go all the way, namely at fullback, while they lack creativity in the centre.
The Scarlets could be dark horses, but it all depends on how their forwards hold up. They went well against a strong Ospreys pack last week, which is a good sign, but the HC is always a step-up. They've got a great backline though, ehich contains big strike runners such as North, Davies and Lamont. Given enough posetion and territory, these guys will cause damage to every team in the competition. Hugely tough group though.