Assume that you can get the follower to wear what you want them to (this isn't a question of "How to do I get Lydia out of her Steel Armor").

Do followers benifit from all enchants or just some of them? I recall seeing once that followers use some enchants (unfortainately I can't find the reference).

For example, if I give Aela a helmet with Fortify Archary, does she gain any benifit beyond the armor?

Yes of course they get the buffs. Why they wouldn't?

yes; equality is very important in Tamriel

On my lvl 81 character I have been experimenting with the alchemy expoits and have made some uber enchanted items as a result. I have given a ring (or necklace, can't recall which atm) with a magicka and magicak regen enchants to J'zargo. Something like 5847 magicka and %5847 regen rate (not in front of xbox, so can't quote exact numbers). I have watched him use "sparks" to kill a giant all by himself. Continuous zapping for a very very long time.

Not everything.

http://www.uesp.net/wiki/Skyrim:Followers#Enchanted_Follower_Equipment

Thanks Stilts! I knew I'd seen this somewhere.

For those who don't want to follow the link:

Enchanted Follower Equipment
Followers will only gain benefits from the following enchantment effects on their equipment:

• Fortify Health
  
• Fortify Heavy Armor
  
• Fortify Light Armor
  
• Fortify Magicka
  
• Fortify Stamina
  
• Fortify Unarmed
  
• Muffle
  
• Resist Fire
  
• Resist Frost
  
• Resist Magic
  
• Resist Shock
  
• Soultrap/Fiery Soultrap
  
• Waterbreathing (Followers can't drown anyway, but this stops them surfacing for air)

All other enchantments do not actually have an effect for Followers, even if they should. Notably, this means:

• Fortify does not increase the amount of damage they do.
  
• Fortify does not reduce enchanted weapon usage cost at all, or remove it at values of 100%.
  
  • There are reports that this effect does work for some Followers, but very few. See the discussing page for more information.
• Fortify Carry Weight does not increase the maximum carry weight.

Now Soultrap/Fiery Soultrap doesn't belong on that list as it is a weapon enchant and, as far as I can tell, followers make use of all of these.