With frameworks like Ext available, the choice of doing a full application RIA-style on the client becomes not just reasonable but quite a compelling option. This style of development does push you to a more service-oriented architecture, which could increase app complexity a bit if you hadn't planned to go that route. However, more and more applications are going the route of services to support non-web (flash, iPhone, desktop clients, etc...) front ends, so I for many apps a service layer is (or will be) unavoidable. Although in some cases it might be more work, it seems that the flexibility it buys is well-worth it.
I'm going to reserve judgement until I have a little more experience, but so far things have been looking really good. I expect that the next year will find me writing more and more and of my web applications code in the browser and less and less of it on the server.