ShinHakkaider
Adventurer
It's not necessary on the user's side (then again, buying a rulebook isn't about necessity or logic for most gamers, but passion and drive). If you're not using Pathfinder's tweaks, you can still run any of the modules, adventure paths etc with very limited conversion work (taking a feat or ability here and there off a stat block, basically). If you use Pathfinder rules, you can still use all your 3.5 material with little to no conversion work necessary (which is part of the point on the user's end).
Paizo needs to have the rules in print, as Erik Mona and others have stated time and time again on these boards and others. Period. That part of the equation on the publisher's side couldn't be more clearly stated. Let's not forget that.
As for splitting the 3.X crowd into "Pathfinder" or "no Pathfinder" camps, I don't see that happening at all. Look at DaveMage above: he still plays 3.5, but does he see Pathfinder as a bad thing? Nope. Me? I'm playing with the Pathfinder rules. I still use my 3.5 materials with them.
What he said.
I'm looking at alot of these posts crapping all over Pathfinder and I'm wondering if any of these people are actually using / or have used to tweaked ruleset? Before I decided not to support 4E (and I was someone who wasnt thrilled about 4E from the outset) I bought and read the rules and more importantly I USED THE RULES TO RUN A GAME before I decided that 4E was not for me. There are things that I liked about it but it wasnt for me.
I'm running a monthly game using Rise of the Runelords as the adventure and Pathfinder as the rules set and I'm having fun with the tweaked ruleset.
The option of having MaxHP + CON at first level are making my players a little bolder in thier approach to encounters. It's also encouraging me NOT to molly codddle my players (even with the higher HP, I almost took the sorcerer PC down near 0 hp with a series of thrown alchemist fire bombs...) The spellcasters are loving the 0 level at will spells, because it means that theyre not running out of spells any time soon. There are things that work
well with this ruleset, there are other things that I've not used (I kept the altered skill list, but used the skill points from standard 3.5. Which works since I'm running a 3.5 adventure and the adventure itself required very little in the way of modification compared to if I had to change it for 4E). But over all it works for me.
Basically if you didnt like 3.5 to begin with (which is the case for A LOT of people here at EnWorld) then pathfinder aint for you. If you liked 3.5 EXACLY the way that it is (and there's nothing wrong with that.) then Pathfinder probably aint for you. There are like a metric assload of 3.5 product out there that can be used for years and years and years so either way you're not at a loss.
For the rest of us who are actually using the ruleset I think that some of us are gonna have problems and some of us are going to love it. The opinions I value are from people who have similar play styles to mine and who are actually using the ruleset. Everyone else's opinion is valid, but ALOT lower on the totem pole.