I feel she is. You'd probably max E or Q and be a tank support. You even have something not a lot of supports have, long range initiation.
I think she's a good support, I'll need to test more once she is free or I have IP to buy her.
khorney2 votesi do it, she is a counter engage supp liek braum and once you have your ult its gg. peopel will say no for 0 reason and when i ask them they give me 0 reason, i do j4 supp and malz supp they call me noob lol people called me noob back in season 3 when i was doign naut supp and look at it now naut supp is a thing lol. when i ask people why it isnt good they give me no reason,
Pyrithegethon2 votesI think it's because people are resistant to change until they see it being good (or a pro using it).
Beldair3 votesFunny, I get that when I do Rumble Support, Mordekaiser Support, Nasus Support, and Swain Support.
People tell me I am trolling and downvote any support threads where I mention that I play those.
warpenguin5552 votesI thinks it's playable but i get a feeling that you shouldnt do it. Cant put on finger on as to why though. I play Malphite and Maokai support and for some reason i'm not confidant in Sej but never got that feeling on those 2. IDK try it out.
Sukishoo1 votesI did that back in season 2 with her, it wasn't bad.
Pyrithegethon1 votesMaybe she's even better now with keystones
Big Lincoln1 votesno
Beldair1 votesAs a support main, I disagree. I've played her in Support and she works pretty good. You just have to know her to be able to bring out her strengths in support.
Maximum Morde1 votesPassable. She's not as good as she used to be when her %hp damage was on her Q, but she's still quite workable.
Ruin Lance1 votesyou could probably make it work, but you can't really take advantage of her whole kit and she would be extremely hard countered by champs like 
Pyrithegethon1 votesWhat tank support isnt hard countered by Janna
warpenguin5552 votes
?
Lux HD1 votesWhat does she offer to a duo lane that Leona doesn't do better?
Pyrithegethon1 votesSpammable aoe CC (her E)
Maximum Morde1 votesLeona and Sejuani support play very similarly, but Sejuani trades a little durability for higher damage and a better mobility spell.
Z Statistic1 votesPointblank R for Sejuani is amazing. Plus, it has a larger stun radius. Leona's has the delay regardless of distance.
Pre-6: Sejuani offers more damage than Leona even with perfect passive procs and is overall more durable Leona has burst durability and more control
However, Leona's main source of durability is both cheap and also compounds on itself. Sejuani's source of durability is comparatively expensive, not as effective later on, and doesn't compound on itself. Sejuani is therefore more gold reliant than Leona, which is bad for support lanes.
Imperial Pandaa1 votesI will probably try it tonight, sounds fun.
Pyrithegethon1 votesLet us know how it goes ^^
Imperial Pandaa1 votesLet us know how it goes ^^
My results may not be considered unless with a grain of salt. My ADC was Lucian, and we laned against a Vayne and Janna. I question their abilities with the champion given. Final score was 4/7/18. The AOE effects she offered helped to shimmer the Shaco jg and Vayne ult. I wasn't sure how to build her really though. (That entire match was weird imo. The comps alone made me cringe.)
AgeOfTheMage1 votesYeah, it could work, she has plenty of hard cc, it sounds like a good idea
Hauling Ashe1 votesShe's OK, but she has a weak early game, which kind of negates your effectiveness in lane until 6. It's easy for an ADC to chunk her when she engages until she can build an armor item.
Pyrithegethon1 votesI can see that happening but who knows, maybe she's better than we think?
Beldair1 votesAs one that plays Sejuani Support, her early game does suffer. But once I get Targon's, I can do what I need to help my ADC.
Me personally, I dash into the ADC or Support to keep them off my ADC.. And 9/10 times, I have an ADC that capitalizes on my dash and strikes the enemy adc.. Yes they lose out on a few cs but we do enough DMG to them that they have to recall which allows us to get a little more xp while they are away.
But I will agree with Titz N Ashe, her true Support Power comes post 6. Her long range initiation is at 6 with her ult. If you want a Tanky Support with a Dash, Stun, and ranged initiation, I'd advise trying Gragas Support.
Escheton1 votesCotC made her long cooldown trades a lot better. As she can now disengage without too much back-end poke. Which was her one weakness. (Other than the actual cooldowns)
And she has quite a bit of dmg and cc on her rotation. Should go well with Cait, Jhin, Jinx. Those with both cc to follow up or just straight up follow up dmg.
AgeOfTheMage1 votesNot sure why the downvotes, they're asking a question and i'd say it's viable
wait, i'm answering my question now:
we're on the league of legends boards.
MunchCrunchLunch1 votesyou'd push wave on engages which is bad also you don't have any ranged way to engage from distance like blitz hook naut Q or thresh. or leona E.
jinxedchef1 votesWhat does Sej do that other champs don't do better? 
, for example, does everything better and with a ranged engage that isn't AOE so it doesn't push the wave.
noir tail fox1 votesIt works great actually, but your adc needs to be a bruiser or tank depending on how you go. I've done it 4 times each with fantastic results. Garden, taric, Darius, rammus, Leona, braum, even rakan, Jax, xin Shao, and jarvan. All can make it go really well. You typically have enough engage to pop and start a fight with their adc, and sej has her passive to keep her safe. Take aftershock and if you get your e in its gg, just bring ignite.
So basically you will want an assassin or burst jungle. Both of you take relic shield and share melee minions and cannons, this keeps you safe to farm and let's you get to three when you have no trouble engaging. If your jungle can get to three before anyone bot can you guys can go ham at two bc they usually have tried going in when I've done this. I've had a yi as my jungle everytime except once. And again the strategy has been 4/4 wins. It's not perfect and obviously not something you'd wanna do ranked, but it's a lot of fun to wreck bit lane and have demolish on both Champs taking lives and towers. Beware of hanks but you can just q backwards and then w while moving to slow any one trying to approach