(Note: The Today Sponge was taken off the market years ago. This was an economic decision and had nothing to do with the safety or effectiveness of the Sponge. It is now back on the market. http://www.todaysponge.com )
Approximate effectiveness: 85- 90% if used properly. Their site claims "89 to 91% effective if used as directed".
No protection against STDs
The "Today" sponge is a soft, disposable foam disc containing the common spermicide nonxynol-9.It is moistened with water and inserted into the vagina. It's immediately effective- at least as effective as it can get, offering both a barrier to sperm and sperm-killing properties. It's easier to use that the Diaphragm or Cervical Cap and the sperm-killing capability lasts about 24 hours
Because of its relatively low rate of effectiveness, the Sponge is not likely to be a good choice for most sexually active Teens for whom preventing pregnancy is highly important. There are essentially no medical side effects (unless you count a nearly 15% chance of pregnancy as a side effect!) However, there may be medical reasons you cannot use some other methods so you should discuss this with your doctor.
This may be a perfectly good contraceptive for couples in a long-term/married relationship for whom a pregnancy might not be sought after, but if it happened, would not be a bad thing.