Typhoon, being an instant, as opposed to a 1.5 second cast, should in theory be more useful for an interrupt, but in actual practice I'm rather unimpressed, both as the caster and as the target on the receiving end of it. Let's take a look at it from the caster's standpoint first.
As depicted above, Typhoon is a big blue blob that travels from the moonkin to the desired target. I have no idea why it looks like neon cotton candy, but it does.
This means that even though Typhoon is an instant cast, it has a travel time to get from the moonkin to the target. All things considered the travel item isn't horrendous since most spells have a travel time to begin with, but most ranged interrupts do not. Counterspell does not. Earth Shock does not. At point blank range Typhoon is likely to interrupt someone due to proximity. There just isn't the time for the target's spell to go off. If the target is at max range, there's a fair chance they'll finish what they were doing unless they have a really long cast or they're channeling.
On the bright side, Typhoon will hit multiple targets so there is the possibility of inconveniencing multiple casters instead of just one, which Counterspell and Earth Shock will not do. There is the knockback effect itself as well, where the knockback might be enough to toss an annoying enemy off a ledge, should the positioning be right.
Since Typhoon is a costly spell (twice that of Starfire) and has a 20 second cooldown though it won't be used as part of any regular dps rotation. It's value is primarily in the knockback, not its damage.
Now, one of the ways I tend to determine how useful a spell is, is by how much it annoys me to be on the receiving end of it. After all, isn't that the best way to gauge how strong it is? By losing to it?
I took Gillien into Arathi Basin, 51-point holy specced, and healing like a madman. Unsurprisingly, there was a moonkin running around. And unsurprisingly (given that I was chain-casting heals like no tomorrow), I was hit by a Typhoon.
I was unceremoniously bumped into the air... and I landed... and I started casting again. Typhoon's knockback is only 5 yards. Honestly, that's barely noticeable. I was hit a second time standing between the Lumber Mill flag and the edge of the cliff, and I didn't fall off. That's how negligible the 5 yard distance is. If moonkin are hoping to knock people off cliffs in PvP, don't count on it unless the person is already standing literally at the edge of the cliff.
Also important to note, the period I was in the air was only a second or two. Probably the length of time of an Earth Shock interrupt, but Typhoon is on a longer cooldown. This means that Typhoon is only really useful when there are multiple targets that can be interrupted. Using it when it only has one possible target to hit is pretty much a waste unless that target has to be interrupted as all costs.
For me, Typhoon is going to be relegated to solo-ing duty or for my amusement (I'm actually casting Typhoon at Onyxia in the screenshot). It might still be useful if I accidentally pull too many caster mobs, but it lacks the precision to be used as an interrupt in an instance, and I'm inclined to think that bosses will be immune to knockback effects. If AoE is needed, good old Hurricane (now free of its cooldown) would work just fine.
There are upcoming changes to the Balance tree before the expansion comes out, and since Earth and Moon and Moonfury are being reduced from 5 to 3 point talents, I expect that I'll get my talent points refunded in the near future. I think there's a good possibility that once I respec I'll end up dropping Typhoon entirely for something else. If I PvPed a lot as a moonkin I would consider keeping it, but otherwise I couldn't justify keeping it.