Big projects, in hierarchies, with large assignments on few (many times one) tasks. The import to MSP process just chokes on this. Better than in the past, but still so slow that it makes gaining adoption difficult. The new XML version on 15.2 will work fine on small projects, but locks up on our mess. Classic still works, but slow.
We did have one group officially switch to OWB. Will see if it lasts as we are losing two key proponents, there.
Keep asking CA to test their MSP Interface updates on our stuff - if it works fast here, expect it will work anywhere fast. No luck, so far.