Actually the percentage of slow used isn't the percentage the delay is reduced. For example, it's been parsed that our best slow (at 75%) actually reduces damage taken by 75%. If this is true, for a mob to only be dealing 1/4 of it's normal damage means it must only be hitting you 1/4 of the time, in other words 75% slow must quadruple a mob's delay. For numerical example, say a mob has a delay of 28. 75% slowing of a mob's delay would be a new delay of 49 (1.75 * 28), but it actually is 112 (4 * 28).
But to the question at hand, I am a barb, and like the other poster, I didn't want to be evil at first. I don't really regret it, I like the fact that barbs are the only race that become a polar bear with our higher level bear form. Of course, that has no effect on play, and, if I was to ever start another shaman, I'd go ogre or troll, most likely troll. Ogre's have the nice feature of unstunability from frontal attacks, which is a big help if you plan on soloing. Trolls get a great necklace (at level 1 I believe) with snare effect, unlimited charges. That's why I probably go with troll, as in my opinion, snare is one of the best spells in the game, pulling is much easier, and not as much worry with runners aggroing friends. I've spent enough plat on serrated bone dirks and balls of burlap yarn to make me think twice about NOT choosing troll if I make another shaman hehe.
As far as role in groups, it was pretty much covered. Higher level shamans are there to buff, debuff, slow, and backup heal, sometimes crowd control in the absence of an enchanter. As far as desirability, at the higher levels, specifically in PoP, where the mobs hit fast and hard, a slower is basically required, as I guess you already know. Of course, chanters get 70% slow, and beasts get 65% (with bst slow also landing a hell of a lot easier than shaman slow), but the extra 5% does add up, and combined with our other abilities, I personally rarely sit LFG for too long.