Hleb just realized he should have still remained in Arsenal. He recently admitted his regret thus:

“I regret leaving Arsenal," Hleb lamented, according to The Daily Express.

"I was playing every week for one of the most exciting sides in Europe. I was being guided by one of the best coaches in the world in Arsene Wenger. I owe him a great deal.

“No player ever gets worse under Wenger. I developed a better all-round game under him. I became more robust physically.”

He left Arsenal rather too soon. He didn't think about the place he was going to. With Messi in form, Henry, Iniesta all favored b4 him did he think he could easily break in? Some how i pity him, but one thing in life we must all take is risk. It didn't pay off well for him, but that is life. I'm sure Flamini would feel the same way. If Adebayor had gone to Barca, he would have suffered more than Hleb.

Just goes to show the grass isnt always greener on the other side. atleast not for everyone.

May be he was wrongly advised. I've been hearing many people say if Hleb goes it would affect Arsenal, bla bla. Even Wenger persuaded him to stay, but he didn't.
Arsenal still remains the best club for grooming talents. I don't think Henry would have been world class if not for Arsenal. Even Adedayo has greatly improved before City asked for him