PROS
-one of the four infinite scaling champions
-Godly late game
-Can deal 500+ dmg in one hit with full tank items
-One of the best AS slows in the game
-Can splitpush and melt turrets in 3 SS hits
-Looks fancy
CONS
-One of the weakest early games
-Requires farm to be effective
-Can be kited hard in teamfights if enemy has cleanse/QSS
-Countered by armour

SPELL ONE:
Use Flash or Ghost. These spells are imperative to get out of tight situations, since Nasus has no real escape aside from W, and that just slows an enemy. Personally, I prefer flash, but in a more drawn out chase, ghost is almost certainly better.

SPELL TWO:
Use Teleport 90% of the time. It helps so much in getting back to lane, and when split pushing, use it to teleport into the teamfight
The only other times i might reconsider is if you have a very disadvantageous lane, I might get heal/barrier. However, if you want to be a bad *** and get ignite or exhaust, go ahead.

Your goal is to outfarm, and outscale, your enemies. Your Q gets stronger every time you use it to kill a minion, monster, or champ, increasing by 3 dmg per minion/small monster kill and 6 per champion/large monster/cannon minion kill. Your goal with Q is not to poke the enemy. If you see them near full, dont use it on them, save it for the minion that is about to die. It might not seem like much, but it adds up over time. A good goal for siphoning strike stacks (SS stacks) Is to have about 100 at 10 mins, 250-300 at 20 mins, and 400-500 at 30 mins. This means that you should not roam, and should not chase 30 second kills into your own jungle. A nasus with 500 stacks will beat a nasus with 0 stacks and 3 full items. (I think :P) Your W is a very potent slow and attack speed debuff. Use it right as the fight begins on the adc, to reduce their damage by 45% if they just autoattack. It really shines in 1v1 splitpushing as well, where you can slow the running enemy to destroy them with a huge SS. Remember to use it after they use somthing like QSS or Gangplank's remove scurvy. Dont use it unless you plan on making use of it in some meaningfull way, because it has a high mana cost. Your E is a large AOE circle that shreds armour and deals magic damage. E's base damage might seem kind of wimpy, but its real strength lies in destroying the target's armor. A whopping 40 armor shred at level 5 can increase damages by SS by over 20%. Make sure to fight people inside of the circle. it has a high mana cost, so don't use it to harass. Nasus's ult is one of the strongest 1v1 ults in the game. A large amount of health is added, as well as increased range and a AOE that steals HP from your enemies around it and converts it into AD. This can make your plain auto attacks hurt alot in a 1v1 after 5 or so seconds. Do not get baited into using your ult. It has a high cooldown and you should not trade that for anything but a kill or an escape from the health. Don't use it until you have made sure that the enemy is committed to fighting you. At the same time, it is benifical to activate it as soon as the fight begins to make the max use out of its %health steal.