Well, let's get something straight first, I'm not the world's greatest mage. I'm far better than I used to be, and I'm actually competitive as a mage now, which I never was, but I'm still not great.
Having said that, I don't use last target. I use current target. All I need to do is have my target set and use current target in my macros, and I don't need to worry about targeting. Casting spells is just a matter of clicking my macro key, or clicking a hotbar icon. And the thing is because I don't need to worry about the targeting I can cast small spells like magic arrow or fireball like lightning, even if I'm interrupted, because all I need to do is hold down a key, or click the icon like mad, and I can click extremely fast.
I can use my mouse wheel to switch between targets, but more often I just click on my target's bar and that makes it the current target and from then on I only need to choose that target again if it goes out of range, or invis's.
The thing is with the enhanced client I can use the thumb joystick on my G13 for movement, something I can't do with the classic client. This leaves me free to use my mouse for targeting, and with the UI mod I use I have a full list of all mobiles in range on screen where I can just click on the bar and it becomes my current target, or I can use my mouse wheel to scroll through them.
Targeting is very easy and I probably acquire targets as fast, or faster, than anyone using the classic client. I don't need to pull health bars, although I can if I want because it can be useful for dealing with stealthers and ninjas especially, but even pulling health bars is easier than it is in the classic client because all I need to do is choose the bar from my mobiles list, and click and drag.