This S90 battery costs less for a reason. It came as a shock when the battery arrived as I had never bought a replacement battery that is not from Canon (for other cameras in the past). However, I saw the price and decided to give it a try in an attempt to save money.
In addition to the price, the battery looks cheap too. It is literally held together by just some duct tape -- which I thought was part of the packaging. When I peeled it off, the inside of the battery (which is never meant to be seen...) was revealed in front of my eyes! Even though I was skeptical, it works fine when I tried it on the camera. That said, I haven't felt comfortable enough to use it for a long time. Therefore, I cannot comment on the life span of the battery.
In hindsight, I should have splurged on the Canon replacement batteries. And for you? I'd recommend to splurge as well.
Usage update (July, 2010): works really well for the past year or so. Its lifespan is pretty much similar to Canon.
Besides some of the writing on the battery, I seriously can't tell the difference from the battery that came with the cannon camera. Look, feel, battery life - no difference what so ever. I typically don't know which one I'm using. I've only had it a few weeks, so who knows if it will last as long. But for $12 when Best Buy was selling the official battery for $40, I'm sure I'll come out ahead.