Leveling my shaman team I am using a very simple focus based system

1 macro
/focus <leader name>
/assist focus

cast normal spells, not in macro

Pros
Setup time was under 1 minute
no thinking involved
if I want to CC or DPS one thing while main is doing something else just target one mash macro main targets new mob
Cons
have to hit the one macro everytime I change targets and want to move DPS
not flexible at all

This will not be enough to PvP or run harder instances - works fine in UK at 68-70.

Now when i was playing my warlocks I tried a focus system and an assist system and they were both much slower the my FTL set up ended up making - I actually documented the process here Warlock & Priest Macro. This was my very first team, ever so there were some thing I missed or would do differently now - so be nice. We actually had a bit of a discussion on FTL vs Focus in my thread too.
My play style was dot dot dot run, new target - dotting as many targets as I could at a time. Before I went FTL on these guys I would often have an alt refresh dot #1 or start with dot #2 on the new mob with dot#1 going into the nether. Of course RAF was announced shortly after I made the group and they were abandoned.

So I think it boils down to what you are doing, how you are doing it and how much time you want to invest in your set up - at least to just get playing.
I think FTL is mandatory for any serious PvP or anytime where you have to switch targets fast all the time.
Focus is easier to set up.