I both play and run games as GM.
What changes do you mean exactly? D&D 3.5 -> Pathfinder or Beta -> Final?
I must say that a lot of changes mean for me that I can get rid of house rules, because not a few of the changes in PF are quite close to (or identical to) what I did in my games.
The new and improved classes will mean I'll see characters of classes I haven't seen previously, or played previously.
For example: I kinda liked the bard concept, but I never quite go around to play one. But now, with the changes to bards, I am playing one, and enjoying it immensely. I don't deal massive amounts of damage, but because of my support, others hit more often, and hit harder, so I effectively deal a lot of damage. I also know everything (or nearly so), and of course, I'm the true hero of the party (because heroes aren't really made on the battlefield. They're made in the tavern, while the masses listen to the minstrels sing epic ballads about great battles. And who's writing and performing those?)
The simplification of the various combat manoeuvres means I'll see and use them more regularly, too.
Changes like the cleric's channel energy (which replaces turn undead) and the fact that you can crit and sneak attack most enemies will mean that many classes are more useful more of the time - especially rogues, who are now a good choice even if the GM announces that the campaign will be chock-full of undead. In 3e, that meant you were playing a non-combatant.
I think the changes to the classes will be the most useful thing for me. That and the Combat Maneuver system. The classes mean I'll see more players playing classes that had only been glossed over before, Bard, Paladin, Monk are all parts of these. My players had touched on them before but they were classes that were only played on a whim, only recently have I had any player look at the Paladin and go "Wow, I really want to play a Paladin now" I've even had a couple grudgingly look at the monk in approval. Then again when I was on vacation I stayed with some people among whom were a pair of power gamers that showed me some rather frightening monk builds which involved multiclassing and prestige classes but still made the DM in me shudder.
I love the Combat Maneuver system, my players have felt it's benefit, as they have it's down sides, but once one gets used to it's concept it's a lot easier to manage than the previous grapple rules.
I dislike a lot of the spell changes and will likely, sadly, be reverting to 3.5 or at least houseruling them to be a little less full of suck.
I like a lot of the feats but felt Power Attack really didn't need a change. It never slowed game down for me even when we had players who sucked at basic addition working with it. Honestly it was spellcasters who slowed the game down sifting through their spells trying to decide which ones to cast that slowed things down more than anything.
All in all I'm glad I have only one monster of a book to reference though I suppose once the Bestiary comes out the two manuals will weigh more and take up as much space as the core 3.5 set :P But at least I can use the PRPG Corebook as a bludgeoning weapon for when I walk alone late at night :D
I dont like what they did with all of the feats. There are also cases where two versions of a feat such as power attack are both valid, but have a considerable difference in power. If I like the 3.5 and pathfinder version of a feat one of them will be Feat X, and the other will be improved/greater Feat X for the most part.
Another issue I am having is pathfinderizing 3.5 monsters. Giving them access to all the feats they are supposed to have may make some of them to strong. I will find out for sure next weekend. 3.5 would just give a monster toughness 20 times, but toughness has changed, and I never liked that idea anyway. Once I get a better feel for the Pathfinder classes I should be able to match monsters up better.
After Edit: Edited due to a misunderstanding of a rule.