Eta had the value of a long mid front vowel (long "e"). German "See" is very close, but for English, the closest approximation is probably "say", which is a diphthong. This refers to Ancient classical pronunciation, before the Hellenistic period.
It is definitely not a hight front vowel ("see"), which is the value it had in Koine and has in modern Greek.
It is definitely not a hight front vowel ("see"), which is the value it had in Koine and has in modern Greek.