The choices are: 

• Gackle bat familiar pet - Strength & Precision buff
• Mouse Droid familiar pet - Constitution buff
• Mynock familiar pet - Agility buff

Some of the questions I have:

• How do the buffs work?
• How much does each one add?
• Can I have it and my beast out at the same time?
• What if I'm on a spy?

http://www.swghuntersguild.com/images/4year.jpg

Once per Account

Invulnerable - cannot attack or be attacked

Follows master - if master gets 120m away the pet will disappear (repack)

May only have 1 familiar out at a time

May have Familiar, beast  and a mount out at the same time

May not be transferred

Familiars are scaled small so as not to interfere in combat targeting

When a familiar is in the world following his master the master gets a buff, this buff goes away as soon as the pet is packed

Gackle Bat  provides Precision and Strength Buff
    Char Level 1 - 19 : +5 Precision, +5 Strength
    Char Level 20 - 29 : +10 Precision, +10 Strength
    Char Level 30 - 39 : +15 Precision, +15 Strength
    Char Level 40 - 49 : +20 Precision, +20 Strength
    Char Level 50 - 59 : +25 Precision, +25 Strength
    Char Level 60 - 69 : +30 Precision, +30 Strength
    Char Level 70 - 79 : +35 Precision, +35 Strength
    Char Level 80 - 90 : +40 Precision, +40 Strength
Mynock provides Agility Buff
    Char Level 1 - 19 : +5 Agility
    Char Level 20 - 29 : +10 Agility
    Char Level 30 - 39 : +15 Agility
    Char Level 40 - 49 : +20 Agility
    Char Level 50 - 59 : +25 Agility
    Char Level 60 - 69 : +30 Agility
    Char Level 70 - 79 : +35 Agility
    Char Level 80 - 90 : +40 Agility
Mouse Droid provides Constitution Buff
    Char Level 1 - 19 : +5 Constitution
    Char Level 20 - 29 : +10 Constitution
    Char Level 30 - 39 : +15 Constitution
    Char Level 40 - 49 : +20 Constitution
    Char Level 50 - 59 : +25 Constitution
    Char Level 60 - 69 : +30 Constitution
    Char Level 70 - 79 : +35 Constitution
    Char Level 80 - 90 : +40 Constitution

If you are a spy and want to stealth you should pack your familiar first.