I did this a few years ago, highly recommend it. Took about a week, spent 2 days exploring Lourdes first. I had done the Camino starting in St Jean previously, but it was rainy and foggy crossing the Pyrenees. I didn't really get to see them at all. So I spent a beautiful week in them, the lower foothills part, rolling hills, farms. Definitely prepared me for the rigours of that climb from St Jean to Roncesvalles.
Oloron St Marie is gorgeous, and Hopital St Blaise, a Romanesque church hidden in the woods on the way was wonderful. You follow the Gave de Pau for long stretches, the prettiest of clear cold mountain rivers.
Some advisos: in 2009 the trail was not well marked. It's a minor route, and I met only one other person doing it, on my last day. I did get very lost on one day: get good maps, and be aware there are alternate routes to confuse things. One of the other Grand Randonee routes through there also uses yellow blazes. It was a very solitary ramble; I was thrilled to meet the hordes of fellow pilgrims starting in St Jean.
Some good hostels. Make sure to check how to get into them in advance.
Lourdes itself is a great site of Catholic pilgrimage itself, of course. My bad ankle and I had a long chat with the healing waters before setting out, successfully. And I was moved to see all the very elderly and very ill faithful coming to the shrine for a last chance.
I did it in September, btw. 42 degrees c one day, then down to 11 and rain a week later in the woods towards Pamplona.
