A Federalist might think the Constitution is the best document that could have been made, under the conditions of the time, to govern the U.S. The Articles of Confederation proved to be a near-failure; the central government under the Articles was too weak. That the U.S. needed a stronger central government was the belief of the Federalists. The Constitution provided a relatively strong government while setting up a system of checks and balances to prevent any part of the government from becoming too powerful and abusing that power.
An Antifederalist, however, might view the Constitution differently. Antifederalists were against the idea of a strong central government. They government proposed in the Constitution, they believed, would trample the rights of the state governments, not to mention the rights of the people. The Antifederalists feared that they would end up in a situation no better than before the Revolution.
This does not mean that all Antifederalists were completely opposed to the Constitution. If only some states were to ratify the Constitution, there would be a divided nation. And a divided nation is prone to many more problems, both from without (foreign domination) and within (lawlessness, rebellion).
So eventually the Constitution was ratified by all 13 states. The Federalists got their strong central government. The Antifederalists were either outvoted or joined the Federalists. So today the U.S. is governed by the Constitution.