The harridan does ok. It won't reliably kill a knight, but should take off a few hull points. Maybe not what you would expect from something that costs nearly 750 points, but hey, it's a tyranid.
Works best teamed up with a couple of flyrants. The shield will definitely be facing the harridan (assuming the knight player has some semblance of sanity), so the flyrants can get some free shots in on another facing.

 JohnHwangDD wrote:
Definitely. It's Forgeworld, so it has to be good.

I hope that was sarcasm.

 Vaktathi wrote:
I think ive only ever seen one of these psychically supported blobs in 7E in person. They're really not what draws most people to IG or how most people want to play the army. They also have a humongous number of common and powerful hardcounters.

Exactly this - the so called "Vunder Blobber" Guard army is expensive, nigh on impossible to build and paint and ultimately useless as just about every army has numerous hard counters to it.

But anyway, back on topic: The basic thing with the Knights is that whilst they are easy enough to deal with on their own, when they come in numbers (IE, entire armies worth) they swiftly become hard to beat without a specialist list.