Hard to make them good without doing retarded downsides and making them a gorramn liability.
- A magic user can get a familiar for free if they want.
- You can make it run around and stuff and I guess it can tell you things that it sees.
- It can hold an extra spell for you and cast it like normal.
- It can't die but you can interrupt its spell casting.
- When you die, it erupts as a Summon spell with HD equal to your level.
Last night one of my players died.
Her cute chicken who only ever used its held spell to cast Unseen Servant so it could pretend to fly around erupted into a 3 HD lightning demon which proceeded to drag one of the other party members back into hell.
Of course it could potentially go well and the surviving players would have a new pet demon for a while, but honestly this is LotFP so it's going to go amusingly bad nine times out of ten.
Also don't tell your players the downside at first. Just let the wizards think they get a free spell slot and be done with it until they find out the reality is far worse.