I saw trickroom use it in one of our battles some time ago (He Punishmented my Bulk Up Palkia to death, but he was only at what, 30%??). Scarf Ho-oh is nice, but there are better revenge killers out there in my opinion. Ho-oh is just too slow, IMO. She can't revenge any common variant of Ray at all (Mixquaza isn't really a sweeper), as she loses to DD and SD (She might be able to survive a +2 LO Extremespeed, only if you pack quite a bit of Def EVs). The Pokes she can reliably revenge are Lati@s, Mewtwo, Darkrai and Skymin, without Scarves. If you look at these Pokes, you can easily see that other revenge killers can do the same thing, while checking things like Rayquaza. Scarf Dialga is an excellent example. He can Draco Meteor pretty much all of them to death, and can use Outrage on Lati@s to bypass their Soul Dew boosted Special Defense, while at the same time, checking SD Ray and non-boosted DD Ray. Dialga also resists SR, while Ho-oh takes 50% from it. Ho-oh can be a good revenge killer, but most of the time, she's outclassed in that role. She's better off using that annoying SubRoost Physical Attacker set.
On the topic of the Physical Attacking SubRoost set, does anyone think she's one of the best tempo controlling Pokemon out there?? If your opponent is spamming Darkrai and Mewtwo and setting up, you can put Ho-oh in there to either threaten it out or kill it, bringing the momentum back in your favour. You can pretty much go "Alright, too fast here, Ho-oh, slow the match down" and she will. On the other side of the spectrum, she can rape a stall team by herself, allowing you to go "Alright, fuck this stall fest, I want this game quickened up".