Because I think I did this wrong on my first outing as a GM, when our group was still trying to run SR4A, let me run down my new understanding after re-re-reading the rules for my personal clarification.

I want to Control someone's Actions. I have a 4 in my Magic rating, and 8 in my specialty of Manipulation spells, and a Force 3 Spellcasting Focus. All numbers in this example are arbitrary, and intended only for the purposes of this specific example only.

1. Spellcasting test: Choose Force 6, roll 15 dice (spellcasting+magic)
2. I roll 8 successes against the standard target number of 5. Since I can't have more successes than Force, my hits goes down to "6."
3. My target rolls his Will+Counterspell of 4 dice, since he can't counterspell, and achieves 0 successes, making my net 6.
4. He is now under my control for 6 combat turns, on the 6th and final turn he may roll Will+Counterspell AND sacrifice his complex action to attempt to break away, or he is stuck that way for another 6 combat turns (as long as I'm sustaining the spell, anyway)
5. Drain test
6. Sustain, at -2 to all other tests as long as I sustain

What happens if I drop my sustain during the 4th combat round? Does he just sit there and drool until he breaks away on the 6th, or breaking the sustain breaks the effect regardless of the "(Force) Combat Turns" rule on SR4A, p210?

. . .

I've also learned that this spell, and other mental manipulation spells should probably never be used against a player, since it just straight up removes player choice entirely. (Unless you're at a con, because con mods are engineered with that kind of crazy in mind.)

I've also learned this this still seems easier than some of the older magic rules I'm used to. >_<

I think that's about it, but by mutual agreement we banned the mental manipulation spells due to the goose vs gander problem you brought up. GM pointed out that the spells were just as overpowered on PCs as NPCs, and NPCs got the same spell selection as PCs, so we agreed to ban them utterly. So I might have missed something.

Oh, and you are overcasting, so you also get to take physical drain. Control thoughts is significantly more effective (mash that "I win" button harder!!).

Yes, the spell ends when you drop sustaining it, and so does the control. You should also note:

1) that any time one attempts to control actions in a way that contravenes the training/ethics/morality/sense of right versus wrong of the subject, the subject gets another resistance test.

2) that the 6 combat turn turnaround is correct. But remember that at every such test, even those in 1) above, successes by the subject reduce the original total of the caster, so eventually the control ends.

Also remember that magic is powerful, and can overpower other aspects of the game, if the GM doesn't use the built in weaknesses and barriers that limit magic.

Technically I seem to remember something about the caster having to spend some form of action in order to control the target's actions (at least with "Control Actions"), and if they don't, then the target is free to act that round. But I could be wrong.

Also note, that with the arbitrary numbers used in this example, the mage is going to be suffering physical drain (F6 spell, only 4 in Magic).

And there's no reason not to just cast it at F7, since the drain value is the same (F/2 is rounded down).