I've played in a group that did slow advancement and it was pretty awesome. We'd play an average of five sessions or so before we gained a level, so it had a more 1E-2E vibe to us (when it was typical to gain a level every three to five sessions or so). It was a blast for us.
But I don't think that everyone would enjoy it that way. It seems that most groups these days enjoy leveling at a bit faster rate than that. Heck, I don't know many groups these days that even do individual x.p. awards any more.
In actual play, our biggest net effect was that we went through disposable magic items (potions and scrolls) a bit faster--more encounters per level and all that. My observation is that around 20% or so of the treasure at any given level is disposable at fast advancement, around 25-30% at medium, and 30-40% at slow. I'd recommend that you discuss disposable items with your GM before the campaign starts, especially if your party is the sort that likes to rely on them. Treasure from individual encounters were also correspondingly smaller, but it wasn't really noticeable over the long haul.