I turn it off.
I don't ever pay to receive SMS (even abroad) and I can send 5000 per month in my allowance.
iMessage is just needlessly complicating something that works fine. I don't have an iPad or iPod touch - and I wouldn't want the messages to appear there.
I don't know anyone who uses iMessage but can't send me an SMS.
The design of any service like iMessage is such that it adds unreliability over SMS.
This is so ill-informed I don't know where to begin:
1. If you did have an iPod Touch or iPad, you choose whether to turn on iMessage on those devices. Heck you can even choose (with iOS 6) if you want to receive iMessages at your iPhone # or emails. That's how granular you can get. Frankly, the ability to check my iMessages on any Apple device makes it awesome. If I am in my living room, and forgot my phone, they still can pop up on my iPad.
2. "I don't know anyone who uses iMessages but can't send me an SMS", yeah that's true because if you don't have iMessage then it goes as an SMS. Not sure how this is even a point? If this didn't work, then they couldn't send ANY other phone a message. So yeah if your iPhone recognizes that the other side doesn't have iMessage it sends it as an SMS. My other thought is, do your other friends all have a rediculous SMS package? My iphone friends appreciate they can send me iMessages since they don't count against their SMS counts (and they can leave those for their feature phone and android phone friends)....
3. How is iMessage unreliable? If there is a problem with the iMessage server or someone isn't readily available then my phone sends it as an SMS. For example, buddy of mine the other day was on a plane with his phone turned off so after trying to send it as an iMessage for a minute or so, it then defaulted to SMS so when he landed it would be delievered by his carrier. Not sure how this makes it "unreliable"....
iMessage is one of my favorite features of iOS/OSX.
EDIT: Oh yeah, and videos. If both sides with iMessage are on WIFI, it sends pictures and videos at their full size and not squeeze them down to what fits in an MMS. That right there makes it worth the price of admission. No more crappy videos shared between my wife and I.