In Julius Caesar, what does Brutus think of Caesar and Cassius?

Brutus respects Caesar, but he worries that he has too much ambition.  He thinks of Cassius as a friend, but doesn’t really respect him.

Brutus’s relationship with Caesar is a complex one.  He has known Caesar for most of his life and considers him a father figure.  However, he had a falling out with Caesar over his actions with Pompey and how he came to power in the civil war.  He is concerned that by naming himself dictator he has taken too much power for himself.

Brutus describes these feelings for Caesar in the soliloquy before the other conspirators arrive.  He explains why Caesar needs to die.  He begins by explaining that he doesn’t have a specific reason for disliking Caesar.

It must be by his death: and for my part,
I know no personal cause to spurn at him,
But for the general. He would be crown'd:
How that might change his nature, there's the question.
It is the bright day that brings forth the adder (Act 2, Scene 1)

Brutus tells the people in his speech after Caesar’s death that he loved Caesar, but had to kill him anyway.

If there be any in this assembly, any dear friend of
Caesar's, to him I say, that Brutus' love to Caesar
was no less than his. If then that friend demand
why Brutus rose against Caesar, this is my answer:
--Not that I loved Caesar less, but that I loved
Rome more. (Act 3, Scene 2)

Brutus feels that the people will understand that he did not kill Caesar just for power and not consider him a killer.  He wanted to be considered a tyrant slayer instead.  He really believed that he was doing the right thing in killing Caesar.

Although Brutus had respect for Caesar, he did not really respect Cassius.  He considered Cassius his friend, but never took his advice.  The only time he listened to Cassius was when Cassius convinced him to join the conspiracy.  After that, Brutus never took his advice.  Cassius suggested that Brutus kill Antony, and not allow Antony to speak, but Brutus did not listen.

Brutus told Cassius to think not of Antony when they are trying to decide who should die besides Caesar.

CASSIUS

Yet I fear him;
For in the ingrafted love he bears to Caesar--

BRUTUS

Alas, good Cassius, do not think of him:
If he love Caesar, all that he can do
Is to himself, take thought and die for Caesar … (Act 2, Scene 1)

This is essentially the same argument he makes when Antony asks to speak at Caesar’s funeral.  Cassius worries that he will influence the people, but Brutus does not listen.  Even after the two of them run their armies together, Brutus and Cassius argue constantly and Brutus doesn’t listen to Cassius.  Cassius doesn’t want to go to Philippi, and Brutus doesn’t listen.  It was a disaster fatal to both of them.