Many people accuse the Fitzgerald translation of deviating too much from the Persian original. Personally, I don't like to see poetry translated from one language to another in verse either, because I will always feel that something has been missed.
However, if it is not translated in verse, then it is no longer has the quality of the original poetry. So what shall we do here?
I think that Fitzgerald has done an excellent job in translating Khayyum. It is said that good poetry has a balance of two things - beautiful language and meaning. Ftizgerald has achieved this.
If you are looking for a more "literal" translation, to get exactly what Khayyum said and thought, then you are better to look to a word for word, unrhyming translation, that has taken care to keep the authentic quatrains only - not all the ones ascribed to him. The "Persian Heritage Series" has produced a good translation like this.
Also beware of "commentaries" telling you that Omar Khayyum was a sufi, mystic, or whatever... and that his verses have special meanings outside of the literal interpretation. It is true that poets in Persia used such imagery as "may" (wine), "maykhana" (tavern), "saqi" (cup-bearer), "yar-e nazanin" (lovely maiden) etc. etc. to bring across meanings of God, and heaven, though this doesn't mean that these things are always implied.
One of the qualities of poetry is that it is ambiguous. It must be recognised that people like Omar Khayyum and Hafez were living in times of religious persecution. If you said something against the established sect, then you could be accused of "kufr" (blasphemy) and punished accordingly. Khayyum himself was accused of kufr, as was Hafez in his time through a line of poetry he had written. Hafez protected himself through the ingenious adding of another line of poetry, clearing his name. Khayyum protected himself because he was a guest at the royal court.