Heighten Spell increases the DC (difficulty check of monster saves) of spells below spell lvl 9 to your highest available spell level.
example: web dc for wiz is 10 + 2 (spell level) + int mod + gear/feat mods. If you got a lvl 17 wiz and heighten web, the dc is 10 + 9 + int mod + gear/feats, which is +7 to the dc.
Heighten is quite expensive (less if you got wizard capstone or the expensive improved metamagic enhancements), but on SLAs its free, so its a must for archmages and sorcerers (who got cheap DC-based SLAs with low spell levels)
Sorcs get +8/+7/+6 on the DCs of their SLAs for free, so they must take heighten
Even on my pale master I took it to increase the DCs of my lvl 7 spells by 2 and of my web by 7.
On a divine caster, heighten is less interesting, soundburst or bladebarrier would get expensive and implosion is lvl 9 anyway, they also got less feats than wizards. Still, extremely caster focused builds would include heighten.
On bards, heighten is uninteresting since their best dc-based spell (dancing ball) is at their max spell level (which is only 6) anyway.
edit: ninjaed