[thejeff]

The Air/Fire boosters are expensive, but if you don't have easy access to communions and they let you cast Fog Warriors or other BF spells it's worth it.

If you've got low level Fire or Air mages, it's really nice to be able to forge the boosters. Even one of each can give you access to higher level rituals, summons and globals. (Air Queens?)

Don't overlook Fire/Death for the cheap fire booster, either.