Taking 5 levels in any magic college is only useful if you want to have all the spells available and it lowers AP cost on the highest level spells in the college as well, and as gGeo notes the top tier 18th level elemental spells are not ultimate weapons, if you use one of them get everyone else in the party to back it up with scrolls, arrows and other spells, if you hit a mob with 1 meteorstrike its hurtful but hitting a mob with 3 Meteorstrike's can be devastating, storm seems to do nothing occasionally and Hail can miss targets so using multiple spells is far more effective, think of those as artillery barrages, Earthquake I do not like because it can effect the friendlies in the party and leaves poison pools.
Oath of Desecration and Wildfire seem to be the best buff spells in the game.
Summon spider brings a loathsome ally to play, it is essential at early levels and for lone wolves.
