The MetaTrader 5 blog has translated a review of the closed beta. It was originally written in russian here. Hopefully the public beta is just around the corner, with one comment on the oroginal russian site suggesting 1st October.
"Practically, everything that many developers wanted from MQL 5 and Metatrader 5 was implemented and realized at the highest level. The major change in MQL5 is the introduction of OOP (Object-oriented programming). Professional programmers have now more opportunities, but those who learned to write in MQL4, can use the MQL4 programming in MQL5 without the benefit of OOP."