I haven't played a Shaman yet, but I felt like piping up to second the notion that the Shaman is insanely cool, flavorwise. Just as an example, I think Warlords are cool. Summoning the spirits of legendary Warlords to lead your party to victory is even cooler.
I'm especially fond of the World Serpent Paragon Path, simply because giant spirit snakes are awesome, and because the PP's best ability (enemies take 10 poison damage when succeeding at a saving throw), happens to mesh perfectly with the level 25 Daily Power that summons the World Serpent itself and forces the target to make a total of 4 saving throws just to shake off all the effects.
In terms of how the Shaman works, mechanically, the general pattern is this:
All the at will powers involve attacking through the spirit beast companion and granting some other minor effect.
The Encounter power's generally fall into two catagories.
1. The Shaman attacks through the spirit beast and adds some other effect, usually powering up allies near the spirit beast or having the beast protect them for one round.
2. A short range (range 5 typically) attack where the Shaman summons a minor spirit to attack enemies and then add their power to the Beast Companion for one round. So the Shaman might summon a frost spirit who blasts enemies with cold and then adds some sort of cold flavored leadery effect to allies through the Spirit Companion.
Daily powers most often involve summoning a powerful spirit that attacks and then becomes a zone, although sometimes said "zone" is effectively a single square conjuration that either bolsters allies or menaces enemies. This means that a Shaman who uses a daily will often have two "zones" (his Spirit Beast and the zone daily) for the rest of the encounter. Multiple dailys can result in several overlapping zones of control.
Some Shaman Daily attacks use save end effects (typically range 10) instead of sticking around for the encounter, but they tend to have nasty aftereffects. Save ends, aftereffect 1, and aftereffect 2 powers are not uncommon for the Shaman.
The 3rd type of common Shaman Daily Attack is the close Blast 5 attack, which almost always targets enemies only and/or provide allies in the blast with beneficial effects. This is the closest the Shaman comes to mixing it up at close range.
Overall, I find the Shaman both very tactical compared to other leaders, and with a very unique style. The Shaman really plays like a laid back guy who lets his spirits do the grunt work while he stays out of the way. The Shaman has little reason to even bother with a weapon (At close range, he can just attack through his Spirit Companion) is basically never going to move closer than 5 squares to the enemy unless he's using a Blast Daily, and doesn't really need to be anywhere near his allies. I think his various short range attacks are there to keep him from just peeking around the corner behind the door every battle.
Unlike other leaders, characters looking for Healing and Buffing had best be cozying up to the Shaman's spirit buddies, rather than the Shaman himself.