I've recently given this a go after having used the Autoglym roof kits previously.
Gave the roof a good clean with Auto Finesse Rag Top Cleaner first, left car indoors for a couple of days so the roof could dry thoroughly before applying I1.
Only time will tell when it comes to durability but so far much prefer this combo to the Autoglym kit.
Worth noting that there is an Anti Bacterial version of I1, slightly more expensive but i went for this as i thought it would help prevent any mould developing on the roof.