Just my two cents, because I know that some initial reactions are "...ugh...character conversion...ugh..."
This is translated from the Indian, naturally.
In the case of Jarmok I did a lot of looking at the PF ranger and variants. As we've mentioned a few times, Jarmok ain't your typical ranger in the 3.5 system. I didn't like the Favored Class feature, so I used a variant from the Unearthed Arcana: "Favored Terrain". I didn't want him to be casting spells, so I negotiated a variant for him that he could use "Wild Shape" per the druid ability. I didn't like the two options on "Combat Style", so I traded out for "Fast Movement".
There are other variants that I built into Jarmok too...like his race.
Now, as I move towards PF, I see that the PF ranger has in its level progression both Favored Enemies and Favored Terrain. I still don't like the Favored Enemy...I always forget it!
In any event, the bottom line for me was that I prefer Jarmok built the way that he is, and there was nothing in the PF ranger (or scout...Jarmok is also multi-classing at the moment) that truly appealed to me...at least, not in preference to what he's already got.
Thus, to update Jarmok, all that I need to do is adjust his feats ('cause he would have gotten an extra one in the PF rules), change his skills (because skill points are handled slightly differently), and slightly modify his attack / defense stats.
For Jarmok, it's really not a big deal. I had to do most of this anyway, since he just leveled up.
I'm not saying that it's entirely painless in this case, but I am saying that it's nowhere near as painful as 2.0 to 3.5 was. I expect that not every character would be as easy as Jarmok, but it doesn't have to be something that will make you consider gouging out your eyes.
From what I've seen, most classes have what I would consider to be real improvements in them. I've specifically been looking at the Rogue (which is much awesomer), the sorcerer (lots of betterments), and the wizard (which is a significant enhancement to the 3.5 version). The ranger isn't significantly different, in my opinion (which means, I assume, that they got it pretty close to right in 3.5), and I didn't take so much as a glance at most of the other classes.
BUT, the transition shouldn't be bad at all, and the mechanics seem to be largely identical to 3.5.