Lots depends on the look of the flyer, message and target market. I do it too. I offer 1 room fully cleaned and protected with the purchase of our minimum service.
The same as if you didn't give that "free" deal... Unless you have a great brand in your city, then it might work alot better. They will think there are strings attached.
I found that the "Free Room" offer made high end people suspicious. When I tried it with a couple hundred I found myself having to convince people it wasn't a scam. I threw the rest away and have never even thought about it again!
This type of advertising never pays for itself right away. You will get a few calls and its how you handle them that counts. This is a wrod of mouth business and it might get you a few more jobs.