The auto-aim feature in World of Tanks is bemoaned as easy mode - by those killed by it - and noob mode - by those driving heavy armor. This seldom used feature forces the tank to aim at the designated target's center point, and attempts to keep the aim there as the tank moves around. I’ve had people accuse me of using auto-aim when I wasn't, to which I've replied, “If your tank died to auto-aim, you’re driving it wrong.” This sentiment is one the community holds in general agreement, relegating normal use of auto-aim to players with poor response time or physical disability. I have since then seen the error of my ways, and I've discovered situations where auto-aim can be a useful tool.
It’s rather not good
Auto-aim will focus on the center point of your designated target and does not take into account any movement or armor considerations. Unless your target is presenting a flat side to you, this greatly increases the chance your shells will bounce. Manual aim allows consideration to be given to weak points - such as hatches and cupolas - and tactical targets - such as tracks, ammunition rack areas, and engines. Auto-aim just aims at the target and stands a good chance of impacting naturally or artificially angled armor.
Auto-aim also does not account for any movement in your tank or the target. If the target is immobile and your tank is carouseling, this won’t matter, but if the target is moving laterally it will not aim with the proper lead to hit. Forcing a better firing solution by driving toward or away from your target instead of across may help, but it makes you easier to hit as well.
Auto-aim is also tricky to turn on and off. It only requires a right mouse click on the target to engage, but the cursor must be unobstructed to do this. You cannot target a subject in hard cover unless you can find an exposed bit to aim at, as the silhouette will not suffice. Distant and small targets are also more difficult to target, as most of us are more used to clicking the left mouse button than the right, and thus our muscle control is more precise with the left button. Turning it off is considerably more difficult though, as it requires a lucky right mouse button click elsewhere. I write lucky because - much like Duck Tape - it is somewhat sticky once applied. Also, since holding the right mouse button will also lock the gun elevation and turret rotation, slips will sometimes engage auto-aim unintentionally.
Except when it is good
I never used auto-aim intentionally until I bought the T-50. I started testing it to leave me free to drive since the most important thing in an active scout tank is mobility, not shooting accuracy. After toying with it for a bit, I theorized it might correct an issue I was having with the ELC AMX that was limiting my ability to complete drive-by shootings. I seemed to miss a lot, even at point blank range when speeding by the target. Because of the turning radius of the tank, I couldn’t get any closer to the target than I was before peeling off or I’d hit it.