I've been using mouse over heal macros since release. But you have to be a bit careful with them. The core problem is NOT loosing your cursor to someone, or reactive healing. The problem is that macros are not operational switches on repetitive casts. And no ability queuing What this means is the following:
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If you have this macro (NOTE THIS I A BAD MACRO):
/macroicon "Cure III"
/macroerror off
/ac "Cure III" <mouse>
/ac "Cure III" <t>
/ac "Cure III" <tt>
/ac "Cure III" <me>
And you cast that macro repetitively healing say a tank. If you tap the macro each time while not casting it will validly run through the list and do the first available action. Which per the above design would be mouse over first, your target second, target of target (usually tank), and then you. Which is the fairly traditional WoW healer macro priority.
THE PROBLEM is if you cast this same macro while casting BEFORE the next ability queue something strange occurs. Specifically the macro keeps running (each line takes X amount of time to run). IF your ability queue activates while your macro is still running the line its at will attempt to execute (and then all lines below). This means that with a traditional long style mouse over heal macro, you run a notable chance of executing the macro incorrectly, and a heal going to the wrong person. I noticed this when I started raiding and had to debug what was going on.
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Consequently I changed to the following (NOTE THIS IS AN OK MACRO):
/macroicon "Cure III"
/macroerror off
/ac "Cure III" <mouse>
/ac "Cure III" <tt>
This adjustment limited my macro to two lines. It meant that if I'm moused over the tank (or have nothing targeted and am right clicking - which gives no mouse over targets) I will always heal the tank. No heals can get sucked up anywhere else due to SE strange design. Now while using this macro to heal prey... if heavily spammed there would still be a small chance for a heal to get sucked off to the tank (if I still have the boss targeted). But generally speaking I've found this completely ok for healing savage raid content. The number of heals sucked off to the tank while healing someone else is virtually non existent.
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The last part of the issue is the ability queuing not working. This part sucks flat out. It does result in some loss in healing or DPS with macros. I'm not aware of any work around here except spamming your heals near the GCD.