I cant speak for anyone else, but as a lvl 30 NIN the interrupt down effect kicked in quite a bit for me. I would definitely recommend this katana to anyone who can spare the time or gil for it.
Also for me the spell interrution rate down is very noticeable! Before this katana I ALWAYS get interrupted by mob hits. With this item equiped this rarely happens... Fantastic!
With capped Ninjutsu, this Katana is a Godsend for its level and beyond. Eventually, its DPS starts to hurt it, even as an offhand option, but as noted in the other posts, un-interrupted Ichi spells make pre-Ni NIN a hell of a lot less painful.
I love this katana. When you're able to pull off an uninterrupted Utsu:Ichi with far-from-capped Ninjutsu despite getting hit, you'll love this katana, too.
Only had 3 spell interuptions last night in 2 hours as a level 35 ninja in Sauro. Champaign. Nice main hand weapon for that level. Went with a Fukuro in off hand and was doing close to 200 Blade: Retsu's.
I remember using this to lvl NIN back when in first came out. Then I recently got it again to take NIN from 37 to 49 in prep for the level cap increase. A very nice piece to have for East Ron (S) sync parties.