You can use a HUD and still take notes.

i know that, im saying that all i still take notes, just dont bother with a hud...

I dunno whether you'd categorise it as a "big mistake," but you are certainly giving up a lot of information which you don't have to. In a game where information is your greatest asset, it seems unwise to had that advantage to your opponents if you don't have to.

K10ofclubs doesn't use a HUD.

Tpirahna does use a hud comprehensively and look at that man's limit results. bing bang bosh.. 2milly in the gas tank. check his blog out. 
That guy from doompoker also is a hud addict and his graph is linear as fuark. 
both think statistical data are a crucial element to the game. 

I know many players from high and midstakes who play without. But the thing is , the higher you go the HUD just gets less important because the player pool gets so much smaller, and so its alot easier to be aware of everybodies tendencies. I do think that if others use and you don't, your giving a small handicap, super small in some cases, but i feel every handicap is too much...