Millenium Lounge is operated by IsBank, I called them and get the information that you can use the lounge by paying 50 TL per person (approximately 25 EUR). They accept credit cards and cash.

I really recommend the Millenium Lounge. There are lots of food and drinks (including alcoholic ones) to choose from, and many different kinds of chairs and sofas so you can sleep if you want to. Wireless is free as well. It's also selected as one of the best 10 lounges by The Street

I had bad memories with Garanti Zone (very limited kinds of food) and Yapi Kredi CIP (food wasn't fresh at all).