I abuse mindprobe hideously with the character I'm playing (when I get to playing

), but then he's a compulsive assenser (with the flaw) and just HAS to know all the secrets.. and yes, he gets into trouble. we've ruled that if the target is conscious, the memories being rifled through all flash through their mind as well, so they can figure it out if they're smart. If they're unconscious then, well, they might dream it, but there's no reason they'd figure it out when they woke up except in unusual circumstances.
Improved invisibility isn't that abuse-able imho - the targets get to resist or take a +8 modifier to spot the character. That's if they don't have heat pits. If they do, then its a +4 vision mod because they can still sense the character. it doesn't stop people hearing either unless you're in the middle of a gunfight. There's no reason it would conceal muzzle flash, so there's no reason shots can't head their way with blind fire modifers, or just use suppressive fire. Then there's vehicles. The FAQ, unless its been updated, doesn't really cover it except to offer suggestions as to how to deal with it due to the varying nature of sensors. We just make a general ruling that against silence or imp-invisibility spells, vehicles take half the modifiers of the spell (but don't resist cos they can't) to represent radar, proximity sensors and all the other gubbins they come with. Works pretty well, specially since the team insist on using both spells on their T-Bird all the fraggin' time.
Barrier spells... for some reason a few years ago we decided to drop the general physical barrier spell from the game. No real reason for it, just a GM in one game said he wasn't allowing it, and would only let you have bullet barrier etc. We found it didn't cause too many problems for the characters, (more for the Gm and NPCs, actually), and it just sort of stuck. I can't even remember his reason! But I don't think they're unbalancing unless you've got a mage who can rack up those 6s and cast it at a high force to begin with. And then drain it at that force afterwards with any left over dice.
Nope, I'm gonna vote for Control Thoughts. its one that very few PCs take (god knows why), and the various GMs rarely due it to players because a: the mages WILL resist it, and the Sams, generally paranoid about mages, tend to have developed willpower ratings up at 5 or 6, with spell defense/sheilding from the mages so they have a good chance too. But if it WAS used I can see it being totally uncool at times (like PCs screwing over PCs, f'rinstance).
If you read the section in I think Threats 2, though, about someone getting controlled by a spirit, it could make her do stuff to which opening a door that you aren't supposed too is equivalent to getting them to just raise one hand on command. True, its a spirit power, so slightly different, but its the same principle completely. The person wanted to stop but couldn't, (failed the resistance check) so just did it, all the time knowing she didn't want to, but afterwards not really knowing why she'd done it - and this person was quite a pro shadowrunner generally by the way it reads. And she didn't really figure it out. So I'd say its not automatic you'd know you'd been magically controlled, though you'd know full well you had done stuff and couldn't explain why.
So yeah, very powerful and probably the most abuse-able spell in the system, but I don't think necessarily obvious to the target after the fact, just bloody suspicious!
And whatever happened to Disregard? Cos that would have been my top vote otherwise. That spell was VERY abuseable.