Is it worth buying the Korg is40?

I am considering buying this keyboard. Alternatively, are there any other similarly priced synths that offer anything more.
Like from Roland or even the Yamaha PSR-730?

It all depends on your needs. The iS40 is a great keyboard, and there are plenty of other great ones, too. You need to determine what exactly you're loking for and then go out and PLAY some of these before you buy one.
Tom