Plane Sailing said:
It depends.
[..]
I'm much less likely to multiclass casters, often the decision isn't planned out well in advance but grows organically with the character.
Same here.
In our group, anyone who played a caster would
never multiclass. They might jump ship to a PrC that offered full spell progression at some point, but that was it.
We had a few players who just didn't multiclass their characters at all, mostly because they wanted to get to certain levels in their main class as quickly as possible; players of fighters, paladins, and monks tended to go this route. A small fraction of those aimed themselves at prestige classes, the rest were just planning on sticking with their main class all the way.
As for me, I mostly played rogues. I'd multiclass a little up front (cherry-picking ranger in 3.0, and/or taking a few levels of fighter) to get some feats or a combat edge, but then I'd stick to rogue levels, and ONLY rogue levels, from there on out. The one caster I played stuck with Wizard through 10th level before hopping into a convenient prestige class. I never even considered a PrC for the rogues I played: IMO, there just aren't many (translation:
any 
) prestige classes for rogues that are cooler than getting to take the high-level rogue abilities.
In our group, multiclassing seems to be much more popular with non-D&D d20 games. Nearly all of the characters in our Farscape game were multiclassed, and the same went for Star Wars d20 (except for the Jedi, who bear the caster's curse). Even the one-shot d20 Modern and Spycraft games we did were done with almost exclusively multiclassed characters. I'm not entirely sure why that's the case, but it is.
--
maybe we're just more open-minded about non-d&d classes?
ryan