I remember when we drafted Apoc for the first time at the local store I play at. When the first pack of Apoc came around I looked at it and thought it would be a nice mana fixer, could be very nice in a sort of 5cU build. By me we now call it the Rishidan Port of Limited. In block draft drafting 4 of the are not very difficult to obtain, and in an aggressive b/g/u build having 2 of these in play can deny your opponent of an entire color for quite a while. They work well with a decks mana curve being that they provide an excelent 1st turn drop, and can disrupt your opponent on turn 2. By me shamans are gobbled up very quickly, how do you all feel about them?
In general i would rather it had some sort of offensive/defensive potential as well. It's timing may be better than dream thrush, but at least the thrush turns sideways (and has evasion to boot).
Not that the shamans ability isn't useful on it's own (although for turn 2 disruption you had better be darn sure what colours the opponent is playing, otherwise you may end up helping.
I think the bottom line for me , however, is that reef shaman isn't really a creature. What I mean by that is that you can't really stick it in a creature slot, or your deck will play like it's very short on creatures, as these don't help creature wars. So the question you have left is: would you rather play these or removal/tricks.
If your draft is going badly, and you have a thrush or two and some other mana denial cards, so you can use a more dedicated strategy.
Remember that port works because it takes up a land slot and still acts as a land.
the only time i would take shamens were when i had a very slow deck already and planned on winning matches 1-0 a lot.