In PvE situations, the best use of this spell is to use [Fire Blast] after gaining "Heating Up" to upgrade it to "Hot Streak!", immediately followed by your instant-cast Pyroblast.
Due to the somewhat long cast time, this can be used to open battles when soloing.
Polymorph/Pyroblast: this tactic is mostly seen in PvP. It works exactly as described: Polymorph the target, then cast Pyroblast. You Polymorph first so that the target cannot deal damage or interrupt you while you cast. Keep in mind that in PvP, many people will use various abilities to prevent Polymorph, so be ready to recast it. This is also useful in PvE questing where you pull two mobs: open on one mob with Pyroblast as normal, Polymorph the additional mob, finish off the first mob, and open on the sheeped second mob with Pyroblast.
Prior to the Wrath of the Lich King expansion, Pyroblast was a talent rarely taken by Fire mages due to its long 6-second cast time and poor damage. In order to incorporate Pyroblast into the rotation and make it an appealing talent for Fire mages, patch 3.0.2 introduced [Hot Streak]. This talent allowed the next Pyroblast to be an instant cast ability any time the mage scored two consecutive direct-damage critical strikes with certain fire spells.
Prior to patch 3.0.9, this spell was part of a technique called the "three-minute mage". It combined three talented abilities, Pyroblast, [Presence of Mind], and [Arcane Power], into an instant attack that dealt massive damage, that could further be enhanced on critical strikes if the caster had also talented into Ignite. It was famous as a nuking trick, often used in PvP. Patch 3.0.9 forced a shared cooldown between Presence of Mind and Arcane Power, which somewhat diminished this technique. With the first talent system overhaul in patch 4.0.1, the ability to talent into this combination was made impossible.
Patch 5.2.0 (2013-03-05): Direct damage reduced by 10%.
Hotfix (2012-09-14):Pyroblast and Combustion should combine to do the expected amount of damage and critical damage.
Patch 5.0.4 (2012-08-28): DoT duration increased from 12 to 18 sec, and is now considered a Magic effect.
Patch 4.3.2 (2012-01-31): Pyroblast damage has been reduced by roughly 6%.
Patch 4.3.0 (2011-11-29): Initial damage has been increased by approximately 26%, and damage over time has been increased by approximately 100%.
Patch 4.2.0 (2011-06-28): Pyroblast cast when [Hot Streak] is not active was doing slightly less damage than the Hot Streak version. Its damage has been increased so that both versions of Pyroblast now do the same amount of damage.
Patch 4.0.3a (2010-11-23): Base mana cost is now 17%, down from 22%.
Patch 4.0.1 (2010-10-12): Moved from Tier 3 to being a specialization ability.
Patch 3.0.2 (2008-10-14): Cast time has been reduced to 5 seconds.