Shaman dps is pretty much right up there with a Rogue, Warrior, or any other 1 dimentional dps class. I'm generally in the top 3 myself (spend alot of time in the #1 spot) in Kara runs, and typically 1-2 in pugs, granted my intentional goal ISN'T to be #1 on dps meters...its to be top 3 in dps AND healing (which I do consistantly too)!
The only problem is that Shaman dps (enh more so than elem) has nearly NO ways to manage there hate production! SO unlike a Rogue's fade, Lock's shatter, Mage(block), Warrior's armor, etc...that help with either hate management or methods to deal with the issue when aggro is taken, Shaman's are completely dependant on the hate keeping skills of their tank, as far as how much dps they can pour on. Without a good tank, said shaman is going to be dead shaman, and being dead kinda hinders your dps :p Keep a shaman on there feet, and let the beating ensue!
This being said, the on 2 seperate occations running a heroic run in SV, the pug'd dps war (arm's both times), both did 47%-53% of the TOTAL DMG DONE in thoughs runs!!! WOW!!! I ended up being #2 by a LARGE margin (I was like in the low 20%), and despite the wars' overwhelming %, I was actually doing my usual good #'s! The war's in question were EXPECTIONALLY geared (tier 5, BT, MGlad sets caliber stuff), and previously no dps warrior really put #'s up that impressed me as these guys did! Now, in my (a enh sham's) defense, the wars' in question, in this situation had the liberty to cleave and whilwind all they wanted! AoE dmg is something a enh sham doesn't do very well, thus when factoring if/when situation gets to happen for the dps classes that can expliot it...then ya...sham's are always going to be beaten. Regardless, thats some pretty INSANE dmg!!
All that dps, and the ability to go all out and not worry about pulling hate.....DAYUM! I was inspired to start my own dps warrior (he's @ 47 now :D), after witnessing that sexy display of brutality, and compairing it to my frustartions of dying each time I yanked hate :P!
So I'm inclined to say, if pure melee dps is your intention...then I'd be partial to suggesting warrior over sham. Course, understand that dps is a dime a dozen! Sham's life (regardless of spec) revolves around healing, kinda like a warrior lot in life revolves around tanking (regardless of spec). So that being said, in the majority of situations that your NOT getting to dps...are you "ok" with the other aspect that comes with the classes in question :P?