PROS:
-EXCELLENT damage output
-very fast farmer
-perfect harraser
CONS:
-very, VERY fragile
-number one target in team fights
-with all spells on cooldown absolutely useless

lets analyze all of the pros and cons:
-damage output: abnormally big damage when used in right way (ill explain that in the combo section)
-perfect farmer: with oneand runes, capable of destroying caster minion wave with 2 spells.
-harasser: throwing just one pillar of flame gets most of the players retreating because of the try to avoid the damage.
-fragile: there is a reason i rush rylai. brand is very squishy character, and his positioning in team fight is very important.
-easily tageted: because of the damage output, brand tends to be the no. 1 target. this combined with the fragility is a very annoying trait.
-cooldowns: with the coolodowns on, brand is pretty much useless. (this can be countered with cooldown reduction)

playing mid lane means you need to recall as least as possible.
that makes the starting items meki pendant for mana regen (with the runes and the pendant early game it doesnt make brand mana hungry, which he is normally.), and 2 health potions for durability.
having a high mana regen doesnt make you manaless, so that doesnt mean you can just spam spells at minions like you dont have a cooldown and a mana cap. ( harass as less as possible but dont forget you are harassing so that the enemy mid champ is in the state of farming incapability.)
if you really have to recall, thats what the teleport is for.
if you can, stop the enemy mid champion from farming for as much as possible, but dont stop farming yourself.
if your opponent has better items than you, and if you think you may get engaged in a fight in which you will leave just ashes behind you, go back and shop.

the passive. the main spell initiaton ability. it drains 2% of targets hp for 4 seconds, which makes the target lose 8% of their hp in 4 seconds, which makes it a great "oh no, you arent going to survive" passive. The damage is affected by both the magic resistance and magic penetration.
the q spell.
sends out a fireball skillshot.
deals 80/120/160/200/240 magic damage (+0.65 damage per 1 ability power)
the w spell.
it blasts the target area after a short delay. (around 1 second) deals 80/125/170/215/260 (+0.60 damage per ability power)
the e spell. sets the target on fire dealing damage.
deals 70/105/140/175/210 (+0.55 per ability power)
the ultimate.
sends out a giant fireball which innitally moves slowly (more about this in the combo section) but unleashes giant damage that jumps from target to target 5 times. (hits 5 times not 6) takes a very brief delay for brand to cast it (probably because of the casting animation, delay is about 0.15 seconds.)
deals 150/250/350 damage (+0.5 damage per ability power)

brands passive is one of his best attributes.
it also triggers second effects on his spells.
on q it stuns the target for 2 seconds, which is in my opinion perfectly enough for getting your whole arsenal on the target.
on w it makes the damage done increase itself for 25%.
on e the conflagration spreads to nearby enemies.
on ultimate the missile speed increases.
Now lets go over the main combos.

when you are going 1v1 or focusing one target, pick the e-q-w combo.
it lets you stun the target, set it ablaze, and do the extra damage with the pillar of flame.

if youre just looking for the stun, go for the e-q combo.
there is a trick with this combo that reqiuers full concentration on timing and quick reacting. first let the q out, then while the missile is on its way, set the target ablaze with e. it takes some practice, but it isnt that hard to do.

if you have just been initiated in team fight, take the q-e-w-r combo.
it lets you set the whole team team ablaze and do the extra damage with pillar of flame, it does not give the enemy time do spread so the ultimate will shred them apart, but it is not going to stun anyone. sometimes it is better to go with the stun combo than the max damage combo.

for farming, first deal some damage with q, then conflagrate the enemy minions.
once again, dont overdo it with the farming.

starting items are explained in the PLAYING MID chapter.
i prefer the magic pen boots, because it works with your passive, but the cooldown reduct boots are a welcome possibillity.

after you get yourand, its a big probability you are going to get ganked. because of that reason, i rush. it makes you more sturdy, gives you ap, and slows your attackers if they are getting a little too excited of your death.
do i really need to explain this? some people rush this item before rylai, but rylai makes you feel helluva lot more of the "im safe" feeling. your choice.
its a especially must-have item if you are in a ap havy team.
it gives you healing from your spells, and bonus ap. any other questions?
i prefer this item much more than the void staff, because the scepter gives you ap, gives you better protection against the enemy caster, and in combination with boots, gives you enough magic penetration against most of the non-tank characters.
is a all-time must have because of its cheap price for the effect.
it gives you a decent amount of ability power, and is a perfect item if you started off a little bad.

RECOMMENDED

in my opinion, teleport is a perfect spell to get back to mid lane fast, and to take care of those minion waves that are herding at your bottom/upper lane, while youre teammates in that lanes are either dead or occupied.
flash is by me a perfect escape and entreance spell. it lets you get over the walls quickly enough to escape from enemy team or to come in to assist your team.
ignite is a perfect spell on brand. getting the full combo on a enemy with ignite is almost always a 100% kill.
even if i dont like it that much, i still recommend it.
i prefer flash simply because of its trait that i can get over the walls with it. but if you are more familiar with ghost, take it.
brand is mana hungry, but with runes and early mana regeneration, you dont need this spell. but if you think you would be better off without meki pendant, and with some other runes, take clarity.
it is a great spell but it just isnt my cup of tea. your choice.

NOT RECOMMENDED

i have seen some people play it, but i would rather leave this spell to some ad character.
leave it to your jungler.
leave these to your tanks.
once again, i saw some people play it, but i think you should leave it to your support.
stop trolling.