**** yeah. Both Zelda and Sheik are. They are great characters, they're not the most populars (another advantage imo..) AND they aren't the most broken...
Is Zelda viable: yes... but play her just if you like Zelda's style.
You can be lethally unpredictable with them;
<3 Zeldina's awesome style and obvious improvement
<3 Sheikita's ninja edge, edgehogging and gimping game, the ****ing needles, and ftilt lock
<3 Transform; making this character what it is
<3 Zelda's fair, bair dair are the nicest moves to watch in brawl; with the time-stop animation and Zelda's dair which is the third strongest spike in brawl =O and obviously uair is the move you can count on to get easy KOs
<3 Zelda's usmash, dsmash, fsmash
<3 Zelda's dtilt lol, utilt
<3 Zelda's dthrow <3 <3 <3 nice animation too
<3 Zelda's ********
<3 Nayru's love is great to put ppl in front of you no matter how you hit them
<3 Nayru's love also piss off Samus and all those who try to shoot ******** at you
<3 Farore's wind, that is one of the greatest thing that happened to Zelda... I mean you can hit twice in one hit, you can travel fair distances, you can use it to switch strategies in a match... I love Farore's wind.
<3 Zelda's taunts, even though that's not the topic.. her waving goodbye is just soooo mean... after a 2-hit ko by fair+dair that oughta hurt
<3 Sheik's usmash***, dsmash, fsmash all great moves
<3 Sheik's ftilt***, utilt, dtilt
<3 Sheik's uair, bair, nair, fair
<3 Sheik's dair fast manoeuvrability for mindgames
<3 Sheik's vanish !!! thx for the buff. I liked to use it as Melee Sheik
<3 Sheik's dthrow
<3 Sheik's needles of course.. 18% of easy damage
<3 Sheik's chain, oh man that thing can hurt now
<3 Sheik's high manoeuvrability
<3 Sheik's ponytail ! Lolz ponytails FTW
EDIT: Sorry for the Zeldina, Sheikita thing...