Yep. To give an indication of the work involved:
For my last 3e session I wanted to prepare an encounter using some of the Mindflayers of Thoon from MM5 and their associated constructs. I was aiming for a ECL14 encounter.
Initially, I wanted to use three different kinds of mind flayers and two construct types. I quickly realized, though, it would be hell to direct them all in combat, so I boiled it down to the mind flayer clerics and the basic fighter constructs.
Then I noticed the mind flayers didn't use the 3e psionic rules (*sigh*) and were terribly weak for their CR (cleric levels should really be non-associated for mind-flayers).
So I ended up creating a new stat block for them including level 9 psionic powers and level 7 cleric spells. Between selecting feats and items, adjusting the stats for long-time buffs, thinking about general tactics, and writing short notes describing spell/power effects, this took me three hours.
I'm pretty happy with the result but it really _is_ a lot of work. I do derive a certain amount of fun from 'optimizing' monster stat blocks, but I still dread doing large amounts of them, especially those involving various class levels.
You really don't want to know how long it took me to create the weapon-of-legacy wielding CR14 noble salamander wizard/master specialist/abjurant champion I created as the BBEG for the session before that...
The most fun for me is thinking about story hooks, plots & twists and cool settings. The work starts when I need to write down at least a couple of notes and outlines for the stuff.
I have to do it, though, since we only meet to play every 3-4 weeks. Without some kind of notes I'd have forgotten everything we'd done in the last session...
So, fun: thinking about stuff & optimizing stats; work: writing stuff down
