Once upon a time, I thought it was okay to be a Qur'an-only Muslim. My 3-yr shahadah anniversary passed just last month, so about 3 years ago I was in the "I am not sure I want to be a Muslim" stage. Shortly after that I moved into a Qur'an-only mentality, since I accepted the Qur'an but didn't understand some of the stickier aspects of Shari'ah and Fiqh. I just didn't want to accept the Sunnah as a source of Islam.
It might be a position that might be unfamiliar to many born'n'raised Muslims, since they have grown up hearing stories about the Prophet Muhammad (saaws), hearing his sayings which directed them towards righteousness and good manners. On the other hand, Muslims who convert (like me) might be exposed to anti-Muslim websites that take ahadith out of context and really try to smear the character of our noble prophet (saaws.) So even though a hadith rejecter might accept the Qur'an and the prophethood of Muhammad (saaws), he might be faced with statements from him which seem, through some twisting while out of context, to be contradictory to Islam.
One tactic of a hadith rejecter is to say that Muhammad (saaws) forbid his Companions (raa) from writing down his sayings. This is true--but of course, where do they find this information? Of course, it's a hadith. So that line of reasoning is flawed to begin with, but I want to try to respond to it anyway in case anyone else has wondered about this. I have heard from several sources that even though Muhammad (saaws) did forbid his Companions (raa) from writing down his sayings, he did later allow them. I haven't found a specific reference for it, though. They also explain the reason for the prohibition--it was not so that the sayings of Muhammad (saaws) wouldn't be preserved, but rather so that they would not be confused or mixed in with the Qur'an, so that the Qur'an would remain pure in their writing of it. And we know that once this was assured, the Companions (raa) were allowed to write down hadith as well.
I think the best way to start to respond to anyone who wants to reject hadith, however, is with the Qur'an, the Noble Book and Speech of Allah Almighty, Lord of the Worlds. When the Lord of the Worlds has bestowed on us revelation, how can we argue with it? And glorified and exalted be He, He has bestowed on us such revelation.
The Lord of the Worlds has said what can be translated as:
4:59 O you who have believed, obey Allah and obey the Messenger and those in authority among you. And if you disagree over anything, refer it to Allah and the Messenger, if you should believe in Allah and the Last Day. That is the best [way] and best in result..
4:60 Have you not seen those who claim to have believed in what was revealed to you, [O Muhammad], and what was revealed before you? They wish to refer legislation to Taghut, while they were commanded to reject it; and Satan wishes to lead them far astray
4:61 And when it is said to them, "Come to what Allah has revealed and to the Messenger," you see the hypocrites turning away from you in aversion.
4:62 So how [will it be] when disaster strikes them because of what their hands have put forth and then they come to you swearing by Allah , "We intended nothing but good conduct and accommodation."
4:63 Those are the ones of whom Allah knows what is in their hearts, so turn away from them but admonish them and speak to them a far-reaching word.
4:64 And We did not send any messenger except to be obeyed by permission of Allah. And if, when they wronged themselves, they had come to you, [O Muhammad], and asked forgiveness of Allah and the Messenger had asked forgiveness for them, they would have found Allah Accepting of repentance and Merciful.
4:65 But no, by your Lord, they will not [truly] believe until they make you, [O Muhammad], judge concerning that over which they dispute among themselves and then find within themselves no discomfort from what you have judged and submit in [full, willing] submission.
I pasted the entire passage just to show the whole context. In this passage we see that Allah has told us to obey Allah, and to obey the messenger (i.e., Muhammad saaws;) to refer our judgments to Allah and to His messenger. And then Allah describes the Hypocrites, who instead of turning to Allah and the messenger they turn to other things which they find more important. They are told to come to what Allah has revealed (i.e., the Qur'an), and to the Messenger. Why to the Messenger...?
But then Allah describes them as hypocrites, the people who turn away from the Messenger. And then Allah reminds us that He sent Messengers to be obeyed! Not just revelation, not just scriptures sent down from the heavens, but Allah (SWT) sent us Messengers as well. Then He (SWT) tells us that true belief and submission is to make Muhammad, the final messenger, who was sent to all of mankind, a judge over their matters.
I ask Allah (SWT) to make us not among the hypocrites, but to be among those who submit to His will, who obey His messenger, and to be counted among the believers. I think this is the better way to begin a discussion with a hadith rejecter: with the Qur'an. And in the Qur'an our Lord has commanded us to obey the Messenger.