Spectacularly creative and fantastically great fun! Getting to see the way that Jack and Ianto react to Hogwarts and how everyone at Hogwarts reacts to them was hilarious and fascinating and so, so well done.
Minor spoilers, all found out very early on. We've found out that Ianto is really Sarin Lestrange, son of Bellatrix. He was resorted from Gryffindor into Slytherin after his parentage was discovered during a potions class experiment, and it alienated him from everything magical to the point he left the Wizarding World behind completely once he graduated Hogwarts.
In book 1, Ianto finds out that Hermione is his (and Jack's) daughter (never fully explained, but attributed to Jack's time-traveling mojo) and when the students at Hogwarts find out that Hermione is Bellatrix's granddaughter, she goes through almost exactly what Ianto did.
Jack and Ianto arrive at Hogwarts just in time to see both Harry and Hermione be resorted into Slytherin. (But not before the rift-activity warning goes off, a dementor appears, and they kill it--apparently dementors are aliens... :P Hilarious!)
At that first dinner, Ianto blithely pulls out his pda and is sending out emails and messages to the team. Everyone is shocked: technology can't work in Hogwarts! /gasp... So Jack rolls his eyes and asks if anyone has actually ever TRIED getting their mobiles to work there. Again, hilarious!
Anyway, as I said about #1, this idea is just brilliant. The Torchwood and Harry Potter worlds are blended just so creatively and it was so much fun to read!