Yes it is very popular. So far I have taken 4 sets of clients to Pamplona from Biarritz. During San Fermin I have at least one transfer a day on this route.
I dropped some clients off this morning at 10:00 and there were still plenty of people staggering around having stayed up all night.
If you would like some more information about the town and the event have a look at my page here: http://www.biarritzairporttransfers.com ... plona.html
I'm going there again tomorrow morning and I'm going to film a time lapse video of the journey. I'll post the video and that page at some point in the next week.
If you do intend to visit, I would suggest start the booking procedure now. The best apartments and hotels get booked up really early.