I've been to Khao San Road a couple of times now, and it's always really good.
They've changed the menu recently, but the Pad Thai is still pretty darn good. Although, the Bangkok Style one has dried shrimp in it. Not my cup of tea, but it was still really good nonetheless.
As always, be prepared for a wait, and definitely don't turn up starving. Our wait time (party of 3, arriving at 6:30 on a Saturday) was 1 hour. But they have a great system of phoning you when your table is ready so that you can grab a drink down the road while you're waiting.
It's definitely worth the wait, just be prepared for it!