What's my best option (using the latest in the .NET framework) for creating a webapp with with a complete ExtJS frontend (all server interaction happens through AJAX with XML or JSON)? Are there any problems with using WCF? The following (fake, but funny) O'Reilly book cover seems to imply so.
I've successfully used ExtJS with "old" .NET JSON Web Services. I tried using WCF DataContracts and all that to begin with but there was too much friction involved.
The one gotcha is that all responses get wrapped in a ".d" property when serialized to JSON. You need to override the Ext JsonReader class to read the .d property as the response.
Coolite as mentioned by Causas (http://www.coolite.com/) is maturing rapidly and is already capable of doing most ExtJS things 'out of the box'.
Data handling is taken care of for most of the 'standard' scenarios you can think of.
If you have any ExtJS experience you can also go in by hand and get your hands dirty on the javascript itself if needed.
I haven't found anything I can't do with it yet for a fairly complex app.