The Caliphate

It's a stated principle of Islamic Fundamentalism that all lands which were once occupied by Muslims should be reconquered; the stated goal is then to return to the Caliphate days.
Nevertheless, according to this  Wikipedia entry, the Caliphate hasn't been well regarded by Muslim scholars:

"The Umayyads have met with a largely negative reception from later Islamic historians, who have accused them of promoting a kingship (mulk, a term with connotations of tyranny) instead of a true caliphate (khilafa). In this respect it is notable that the Umayyad caliphs referred to themselves, not as khalifat rasul Allah ("successor of the messenger of God", the title preferred by the tradition) but rather as khalifat Allah("deputy of God"). The distinction seems to indicate that the Umayyads "regarded themselves as God's representatives at the head of the community and saw no need to share their religious power with, or delegate it to, the emergent class of religious scholars."

"In fact, it was precisely this class of scholars, based largely in Iraq, that was responsible for collecting and recording the traditions that form the primary source material for the history of the Umayyad period. In reconstructing this history, therefore, it is necessary to rely mainly on sources, such as the histories of Tabari and Baladhuri, that were written in the Abbasid court at Baghdad."

"Modern Arab nationalism regards the period of the Umayyads as part of the Arab Golden Age which it sought to emulate and restore. This is particularly true of Syrian nationalists and the present-day state of Syria, centered like that of the Umayyads on Damascus. White, one of the four Pan-Arab colors which appear in various combinations on the flags of most Arab countries, is considered as representing the Umayyads."

Theological disputes concerning the Umayyads

Sunni opinions

"Sunni opinions of the Umayyad dynasty after Muawiyah are typically dim, viewing many of the rulers as sinners and the cause of great tribulation in the Ummah. For example, in the section concerning Quran 17:60 in the exegesis by al-Suyuti entitled Dur al-Manthur, the author writes that there exist traditions which describe the Umayyads as "the cursed tree". There are some exceptions to this -- Umar ibn Abd al-Aziz is commonly praised as one of the greatest Muslim rulers after the four Rightly Guided Caliphs."

Shi'a opinions

"The negative view of the Umayyads of Shiites is briefly expressed in the Shi'a book "Sulh al-Hasan". According to some sources Ali described them as the worst Fitna."