Well, there are hundreds of threads about it, but it is a frequently asked question so without a search function it is a given that the question comes up every now and then.
Condensed forum consensus:
- brigades per se are less useful than building new divisions instead
- brigades are useful to strengthen powerful and expensive units such as tanks or commandos
- the single most important brigades are Art and SP-Art
- SP-Art is most useful on armoured divisions as they reduce softness even more (percentage-wise) than on an infantry div
- Art is great for units with limited numerical strength like marines and paratroopers
- AC is useful for Mot as it makes them harder and adds org, which means the Mot will remain operative longer (and thus can better fill its role in helping the tanks exploiting breakthroughs)
- Esc-F are a must for Tac
- AT, SP-AT and Eng are mostly useless when compared to alternatives
- brigading foot infantry is only advisable in very rare situations (if manpower/IC are limited)