Both Masem and rbc make good points, but as in everything else to do with programming, there is no one solution fits all and there will always be tradeoffs involved.

If you're writing a Web application that requires a good deal of validation and you implement none of it on the client side, then you better expect to take a considerably greater number of hits on your server than had you done a few simple checks on the client side. Not to mention floating all that data back and forth between the two and aggravating the hell out of end users with a slow connection.

We shouldn't lose sight of the fact that customers pay our salaries and alienating them for the sake of programmer convenience is not good business (unless you happen work for Oracle).

--Jim