Implementing Microsoft Ajax should be easier
I find the whole Microsoft Ajax library powerful but still messy to implement.
It is very hard to manage all the events, trying to not get lost between what belong to the client side and what should fire a server side event.
It looks like this technology need to mature a lot and I expect that with Orcas, we should have something really usable.
One positive thing however, and this is why I stay with Microsoft Ajax, are the Ajax control toolkit.
It's true that the lists of existing controls is already impressive, and doing the sam controls out of the library would be a waste of time.
I am not totally sure, but I believe those controls will be delivered as standard in Orcas.