So, tanks build the tank boots that they need ( The mr or armor), Ap carrys build mage boots, carrys build serker's, supports use the moby boots or some times the Ionian one, and other utility mages buy the Ionian ones too. On who should I buy the Swiftness ?
'nuff said.
Boots are typically a situational choice. While there are certain choices that you almost always get, like zerks on an adc, most of the time you should choose your boots based on what you're against. Some times merc treads are a good choice on a mage if you're playing something slow scaling against the likes of a 
Boots of Swiftness are good if you're against a team with slows but not much hard cc. They're pretty good picks on champions prone to getting kited or who really benefit from movespeed: 
etc.
Hec and Singed are the only ones that I would consider buying swift boots on. Hec because you want the additional move speed during a fight because of his passive and Singed because you want to kite with your poison trail, and it would slow the mobi boots down since you're dealing damage to someone.
i buy them on volibear since i get kited all day lol
Upvoted for the grammar.
Boots of Swiftness are rarely bought, but they are decently strong on Champions who benefit from higher in-combat Movement Speed, like Sejuani. However, the reason why it is rarely bought is because it does not offer other combat benefits like resistances or defensive passives.
However, there are situations where it would be good to buy Boots of Swiftness, such as vs. Team Comps with primarily slows.
I would say noone but
Everyone. Right so, I main support and I like playing support, so instead of 
I get 
instead. Unless they have a lot of hard CC in which case: 
.
But anyway a lot of top laners do well with them, as well as tanks.
Whom*
Anyone who would take mobility boots can also consider Swiftness, especially Swiftness with the Alacrity enchant. You roam a bit slower, but you're way faster in-combat.
Champs who need to stick to targets in melee range to be useful can use all the MS they can get. e.g. Udyr, Volibear, Singed. Maybe Trynd, although people tend to go full glass cannon on him with zerker greaves.
Vi and Varus have some extra synergy, since it reduces their self-slows. The slow reduction also stacks with Sejuani's passive slow reduction, and when applied on top of tenacity it makes you very hard to stop.
In general you'd probably look at swiftness boots if you're ahead and can roam/chase frequently -- if you're behind you'll probably want to be tanky.
I like swifty's when people play ashe, or anyone with slows and im an adc who doesnt want to get caught.
only about 3 champs ive had success with swifties on those would be singed udyr and jungle hec. i hear they used to be pretty good on nasus as well.
Proceed to obliterate the enemy team.
mostly champs that are not afraid to get into battle and r confident to get out of it alive..!!!! like if u have a stacked nasus but u r being held down cause of kiting or summoner spells. it would be a good choice to get swift!! or a tank garen he really needs boots of swift...!! I myself use swift shoes for blitz and thresh cause of people running out of their hooks. it really improve their chances to hook someone!!!
I like using swift on Ashe or against Tryn
Try it on Vi sometime, it's kinda silly being able to outrun people while charging up your Q.
I get them on Carries if I need to kite more safely, tanks if I'm getting kited, and supports (because I hate mobi boots, they are useless in comparison to swifties).
No one.
I like them on Shen since his base move speed is awful and he heavily relies on landing his short-range, skill-shot dash to do anything useful. Boots of Swiftness make it possible to get into a position where that dash can be landed.
c the point you all R making, thanks
Well they are good against teams that just have a ton of slows. I will replace Mobis with those in a heartbeat against teams that lack hard CC
on basically anyone if the enemy tema has a lot of slows or anyone that uses movement speed (rammus hec singed voli udyr)
