Rockbox doesn’t really make any mp3 player run any “smoother”. And as far as problems? It depends on what problems you are referring to. I’ve had my Clip Zip since it came out, and I haven’t had ANY problems with it. And I know many others who have had virtually trouble-free performance from theirs as well for just as long. Most of the problems you read about here on the forum that people have are self-induced, either through ignorance, laziness or lack of care.
What Rockbox does is offer tons more options and settings allowing greater variety and flexibility in how you use the player. Some people find this advantageous, others prefer the simplicity of just pressing the Play button and listening.
And yes, 3rd-party soft/firmware will void any manufacturer’s warranty, including Sandisk’s. Having said that though, if you do need to return to SanDisk under warranty, you can probably un-install Rockbox from it in most cases.
However, Rockbox has been around for years, is updated very often, and stable on a great many different players. Frankly, I’ve never heard of a case where SanDisk denied a warranty claim because Rockbox had been installed on a player.