I actually just finished a Teaching Company tape on Paul by Bart Ehrman, and I’m almost finished reading a book on him as well. I’m not an expert, but here are some thoughts:
First, the web site the original poster cites is a Muslim site designed to attack Christianity. I have no doubt the quotations on the site are real, but they are hardly a representative cross-section of opinion, and they’re short on theologians (does Albert Schweitzer count?). Bishop Spong and George Bernard Shaw are not exactly unbiased. The standard Muslim line of attack is that Christianity as we know it today is the religion of Paul, who corrupted the teachings of Jesus.
Next, you have to read Paul to understand what he’s about. He sees himself as the apostle to the Gentiles, and he’s almost constantly defending Gentiles against Jews who want to make Gentiles into Jewish Christians who obey all the Jewish laws–particularly circumcision, diet, and keeping the Sabbath. So he has to find a way to show that Gentiles don’t need to become Jews to become Christians. This leads him into the idea that Jesus’s death made everyone (including Gentiles) “righteous.” That meant that Gentiles didn’t have to follow Jewish laws; in the past Jewish laws made Jews “righteous,” but that day is over–it still applies to the Jews, but it’s unnecessary for others. In fact, Paul goes to far as to say it is counter-productive for Gentiles to follow Jewish law since Jewish law distracts from the real truth–that Jesus died for our sins, etc. At one point he even equates the old Jewish law with sin.
Paul says almost nothing about Jesus. I think there are 4-5 brief mentions of facts about Jesus in Paul, and there is nothing original there. No stories, parables, etc. You could explain that in multiple ways. Maybe Paul didn’t know much about Jesus aside from his own personal vision–after all, he doesn’t meet the other apostles until three years later. Maybe Paul felt there were a lot of well-known stories about Jesus going around, and he didn’t need to tell them himself. Maybe it didn’t come up because his epistles are written to respond to various problems in congregations, not to summarize the story of Jesus.
Paul claims apostleship from his vision of Jesus, whatever that may have been. Acts gives contradictory versions–in one the others present see Jesus but don’t hear him and in the other they hear him but don’t see him. But clearly something major happened to make Paul switch sides.
Paul also seems to get himself into contradictory positions, or at least he is sometimes misunderstood by his followers. He doesn’t systematically think through what he’s writing. When the people of Corinth became Christian, Paul taught them they were somehow “saved” by the death of Jesus, and the Jewish law didn’t apply to them. So they took that as a license for debauchery–almost a Gnostic view that the body didn’t matter. So Paul had to write 1 Corinthians to get them back on track and assure them that their actions DID matter. But since Paul was expecting the end of the world momentarily, he dismissed a lot of things–like marriage. Why get married if the world ends tomorrow? Etc.
Where Luther and the Reformation went wrong is to take Paul’s statements about the death of Jesus making everyone “righteous” out of context. The context–again–was an argument to show that Gentiles didn’t need to become Jews and follow the Jewish laws to become Christians. The death of Jesus made them righteous, not the Jewish law. Somehow Luther missed that part, although Paul goes on and on about it.
So yes, in a sense Paul and the Gospels are complementary. In another sense, Paul foreshadows the Gospels because Paul clearly thinks Jesus is God; and since Paul wrote about 20 years before the first Gospel (Mark), and about 40 years before John, where the divinity of Jesus is really stressed, it shows that the doctrine of the divinity of Jesus existed c. 50 AD when Paul was writing. Whether it was commonly accepted or even discussed at that point is another story.
Almost half of the epistles attributed to Paul probably weren’t written by him. You can give arguments from the vocabulary used, the style, anachronistic facts (like pre-supposing a church structure that didn’t exist in Paul’s time or a bias against women–when Paul clearly relied on women–he named them–for various important tasks), and theology (there is a reference to Gnosticism which was probably not around at the time of Paul).