My preference for familiars is for them to be animals imbued with power, gaining a level of senescence, and thus have their own personalities. They're bound to do what they're commanded to do, but will do it in their own way, never sacrificing themselves. They're slightly smarted than the average beast of their type, but cannot speak (caster understands general feelings and mood). Chain pact familiars are slaves, and they're going to react to their master based on how their treated. They might be forced to do a specific action, but they're free to do negative things in retribution (such as destroying treasure, attracting attention, etc.). This encourages the PC to treat them well, rather than abusing them.
As for usefulness, I feel they should be a double edged sword. They should grant some benefit based on the type of creature, with only predators being useful in combat. However, not only would you lose the benefit if they die, but you suffer an additional penalty as well. They should be slightly harder to kill, but also not something that should be easy to replace (maybe a week casting time for the spell). They don't have an initiative, but act simultaneously with their master.