Thank you for the introduction, LNO. Though I would love to pontificate for miles on end, I will keep my response extremely brief. I will leave plenty of room for LNO to contribute his knowledge of the Arab accounts of the Crusades, of which I am largely ignorant.
The Crusades are commonly referred to as successful failures.
They did not achieve their stated goal, expressed in the Clermont Address by Pope Urban II in 1095. They did not achieve a permanent Christian kingdom in the Holy Land, nor did they perpetually open Christian holy sites to pilgrims. In fact, the Crusader Kingdoms of Antioch, Acre, Jerusalem, etc. were all destroyed and the holy regions closed to Christians for centuries.
All of these things nonwithstanding, the Crusades were some of the most brilliant ventures in the history of the western world. They were a profound statement of political expansion and cohesion which emerged from a warlike, fractious, and ultimately uncooperative society. They were perhaps the first sign that the west was catching up to the Byzantine and Arab east in social engineering, military skill, and intellectual culture. It was the first time since the middle Roman Empire that western Europe was able to assert itself politically over extremely remote regions.
There were, of course, serious disasters. The Battle of Hattin. The Children’s Crusade. The list of botches is long. But that in no way diminishes the ultimate effects of the Crusades.
Rather than cite a laundry list of successes, I will address some of Eve’s doubts.
Absolutely not. There was no slavery in western Europe during the Middle Ages. Warriors and peasants alike brought home a vast quantity of luxury goods on each of the 8 Crusades. This stimulated both local and regional trade to a tremendous degree. When the landed aristocracy recognized that certain of these luxury goods had become indispensable, they were willing to pay exorbitant prices to have them imported. Observe the mercurial rise in wealth and power of the Italian cities, especially Venice and Genoa.
One thousand percent yes. This is the subject about which I am best informed. But I’ll keep it short.
One of the greatest periods of intellectual growth occurred in the twelfth and early thirteenth centuries when sophisticated Aristotelian logic, imported from the commentaries of Arab scholars, especially Avicenna and Averroes, was integrated into existing scholastic theology and philosophy. Logic was the rage of Europe. Major university centers experienced an explosion of intellectualism that cannot possibly be understood in our own dumbed-down, anti-intellectual era. Imagine logic games, word play, and grammar being as popular as every corp rock and boy band in America. That was 12th century Paris. At the source of this groundswell were manuscripts recovered from the east.
Keep in mind that the Crusades were not two hundred years of constant, scorched-earth warfare. There were only 8 actual military campaigns between 1095-1291, many of which were on an extremely small scale or were prosecuted by children. It is unwise to overestimate the actual level of hostility. For example, the great Saracen general Saladin and King Richard I of England were notoriously close friends.
Departments of Russian literature did not suddenly shut down during the Cold War. The same is true during the Crusades. Scholars, theologians, and manuscript hunters probably didn’t give much of a damn who was fighting whom as long as they had access to Aristotle, Plato, Cicero, etc. Hell, there is a famous document called Dialogus inter Christianos et Saracenos jointly authored by Christians and Muslims in which they beliefs of both religions are laid out and discussed from Christian and Islamic points of view. It was written during the Crusades.
As will I. 
It doesn’t seem that many crops could have been imported given the vast differences in climate.
No, but increased contact often yields increased understanding. Western Christians also increased contact with the remains of the Byzantine empire and benefitted tremendously. I think that a global perspective is much more sophisticated than merely local thinking.
After all, the Muslims had their chance to kill Europeans and plunder their countries until Charles Martel stopped them in 732. Hell, southern Spain couldn’t get rid of the Muslims until the final conquest of Granada at the end of the 15th century.
Although I am not going to argue that the killing of nobleman was a good thing, I find your response to be really out of line. No one is trying to be a class warrior here. But the fact is, the elimination of second-son-syndrome had a stabilizing influence on European society. By the 12th century, when primogeniture had finally become a nearly universal standard, the only way a younger son could support himself in the fashion that he felt he deserved was by carving out his own fortune. Which inevitably involved a great deal of banditry, robber-baroning, and general thuggery. By giving this element room to grow in a foreign country, Europe essentially purged itself of some massive destabilizing forces. And a lot of second sons benefitted. Either they became lords of the nascent Crusader kingdoms in their own right or they brought back enough riches to live well for decades. A lot ended up dead, too.
There is plenty, plenty more. If anyone is interested, I can post something of a bibliography on the Crusades.
Your turn, LNO.
MR