I have the optio S and it is one of the best cameras I have ever used. The main selling point is its size. It is the smallest digital camera with a optical zoom. It is so small it will fit in an altoid tin. Not only that but the quality is excelent, definitly comparable to the other top cameras there. You may pay a slight price premium for the size but you will find no better.
There are actualy 3 models avalable.
Optio S - 3 megapixel camera (came out last summer yet still no other has come close to its size)
Optio S4 - this update to the S came out in Winter, basicly all it changed is gave it a 4 megapixel sensor, everything else is basicly the same.
Optio S4i - This is the newest, it came out about a month ago. It is basicly the same as the s4 only the design changed a bit (same demensions though) and they enlarged the lcd screen. One thing to note about the screen though is that its is the same resolution as the older S and S4 only bigger so you wont be able to see any additional info, just see it bigger.
If cost is an issue try to find an S or and S4 which you should be able to get around $300. Oh and this camera is actualy smaller then my brand new V600 cell phone, so you know its got to be small.
Oh and to get reviews on any cameas definitly check out the dpreview mentioned by epj3 and also check out
http://www.steves-digicams.com/ which has some of the most detailed and informative reviews avalabe.