It really depends on the exact usage. If it is to be run on a Internet website with lowish traffic or lots of hardware to throw at it then post back is fine.

However if it is a high volume Intranet application then it can make sense to push as much code to the client side as possible. In an Intranet you have control over the client, so don't need to worry about Javascript/DHTML compatibility problems. Moving code to the client side cuts down round trips to the server and can really help things scale better to thousands of users.

I personally don't like the postback design of ASP.NET, it is aimed at people who don't know any better. It fine for small stuff, but you need to be careful if you want it to scale, you can make a real mess if you don't understand the implications of repeated postbacks.

That said you can also create a steaming pile of poo with client side Javascript...

I write big Intranet apps, targeted at IE6, using lots of Javascript and XMLHTTP calls to send data back and forth to the server.