I'm actually really fond of Lore of Heavens, but I'd agree with the other posters. The other lores are almost always superior choices though. However, as was mentioned, fielding Tettoeko lends some definite weight to a Lore of Heavens Slann.
Along with the better spells, also consider the lack of Magic Missiles in Lore of Heavens. That negates the Channeling skill. While Channeling isnt mind bogglingly good, it can be a really nice tool if you are going magic heavy. Any lore but Life and Heavens can use Channeling. One really nice application of it is Hunter's Spear from Lore of Beasts. If you can get a Skink Priest into flank line of sight(hello Cloak of Feathers), you can cast a bolt through a unit's files instead of ranks at a STR 6 hit with no armor save.